Parasite in Love-Chapter 4: This Wormy World

Kousaka put on gloves, leaned back deep in his work chair, and opened a magazine. As one might expect, it was a science magazine concerning parasites. The cover read "The Journal of Parasitology," and the contents were all written in English. Kousaka was impressed. So she could read English as complex as this at her age.

Flipping through, he found a page with a tag on it. A paper by Norman R. Stoll, titled This Wormy World. Kousaka wasn't sure how to translate it. Did it mean a worm-eaten world? Or a world that was like a worm? No, he couldn't forget that this was a paper on parasitology. So then, perhaps "this world brimming with parasites" might be most correct.

The sound of showering from the bathroom ceased. About five minutes later, Sanagi appeared, changed into pajamas. Seeing her with a black towel on her head, Kousaka muttered a "huh" of some surprise.

"What is it?", Sanagi asked.

"Oh, it's nothing big... I just thought that with your silver hair covered like that, you looked like a normal girl."

Sanagi's eyes flickered, and she pointed to her head. "Oh, this? Sorry for being a non-normal girl."

"I'm not saying your hair is bad or anything. It just felt fresh seeing how it would look black."

"I figure you like girls with black hair, fair skin, polite and without any piercings anyway, Mr. Kousaka," Sanagi said maliciously, sitting cross-legged on the bed.

"I didn't say anything like that."

"Then how do you explain all that stuff on your computer?"

"...What do you mean?"

"Kidding. Just wanted to tease you."

"You can't make jokes that ominous." Kousaka sighed, leaning back.

Sanagi suddenly noticed what was in his hand, and her eyes widened. "Hey, that magazine's..."

"Yeah." He'd completely forgotten about the magazine until she pointed it out. "Sorry, I'm always just curious what you're reading. Should I not have gone and touched it?"

"Not really, but... What did you think of it?"

"It was a little tough for me. Are you good at English, Sanagi?"

"Nah. I don't get very good test scores."

"But you can read academic papers?"

"Only in this one field. They all have similar composition, so I've gotten familiar with it."

"That's a big deal. I'd like to tell that to a slacker college student." Then Kousaka aired the question he'd had earlier. "By the way, how do you translate this part?"

Sanagi stood up and came behind Kousaka, looking at the part he was pointing at over his shoulder. The sweet smell of shampoo tickled his nose. She was at a distance he'd normally back away from, but since she'd just showered, it was fine.

"You're an adult and you don't know something that simple?", Sanagi said teasingly.

"Adults aren't quite the fantastic creatures you think they are," Kousaka retorted. "So what does it mean?" 𝒇𝙧𝚎𝐞𝙬𝒆𝑏𝑛𝑜v𝚎𝒍.c𝑜m

"I think it was translated "this world full of insects" in a book I read once," Sanagi mused, as if searching her memory. "It was famously used in 1947 by Norman Stoll to describe the world as one where parasitic diseases run rampant."

"That's one horrifying phrase." Kousaka furrowed his brow.

"Incidentally, even over half a decade later, the situation is almost unchanged. People all around the world unconsciously have many types of parasites in their bodies. And Japan's no exception. Certainly well-known parasitic diseases like ascariasis, schistosomiasis, and malaria have gone away, but parasites naturally lurk in many places in our bodies, waiting for a chance to infect. Or maybe they've already infected, but the person never even realizes it."

Kousaka sighed. "Sounds like the mind of a clean freak will never know rest."

"Unfortunately."

Sanagi said she'd go dry her hair and left the living room.

Since that day they told each other about their disorders, Sanagi had begun taking showers before she entered the living room. Kousaka said she didn't have to show that much forethought, but Sanagi said "It's my prerogative, right?" Once she'd washed, she'd put on a fresh change of clothes she brought, enter the living room, lie on the bed, and read, and talk to Kousaka if she felt like it.

After returning from the bathroom, it seemed Sanagi still wanted to talk, so rather than lie on the bed, she sat across from Kousaka.

Then Kousaka asked her. "It looks like you're always reading books about parasites. What is it about parasites that captivates you so much?"

"...I could tell you, but you might feel sick and faint, okay, Mr. Kousaka?"

"If I hear it in this room, I think I'll be okay."

"Let's see..." Sanagi put her hand to her chin and pondered. "Mr. Kousaka, have you heard of Diplozoon paradoxum?"

When Kousaka shook his head no, Sanagi began explaining the ecology of that parasite. Their lifelong copulation, their appearance coming to resemble a butterfly, their fated love at first sight, the blindness of love, worms that were two peas in a pod. After talking for a while, Sanagi suddenly became aware of how talkative she was being and her face reddened, but Kousaka said "Keep talking," so she talked a little more.

"This earring." Sanagi parted her hair to show Kousaka. "This is modeled after a parasite too."

"It just looked like a blue flower to me. So there's a parasite shaped like that?"

"Right. It's called Kudoa septempunctata, and it belongs to the Myxozoa. These parasites use both fish and annelids as alternating hosts, and each of their spores has six to seven flower-petal-shaped structures called polar capsules, so it looks like a full flower from above. The D. paradoxum keychain is a little exaggerated, but if you dye K. septempunctata blue, it really does look exactly like this earring. Look it up."

As instructed, Kousaka did an image search for "Kudoa septempunctata" with his smartphone. And indeed, many images came up of tiny organisms under a microscope that looked just like Sanagi's earring.

"The spitting image, right?"

"I'm surprised there's a parasite this pretty."

"Well, it causes food poisoning, so it's a harmful one to humans."

Kousaka put down his phone. "Are there any more interesting parasites like this?"

"Hmm, well, let's change course a little for the next one." Sanagi folded her arms and thought for a while. "Since you're such a clean freak, even without studying parasites, I'm sure you know Toxoplasma gondii?"

"Yeah, naturally." Finally, a name he recognized. "They're parasites that transmit from cats to humans, right?"

Sanagi nodded. "Yeah. They're famous as the cause of toxoplasmosis. Their final hosts are cats, but they can infect most warm-blooded animals, which of course includes people."

"Final host?", Kousaka asked, an unfamiliar term coming up quickly.

"The host that's the parasite's final destination," Sanagi explained in layman's terms.

Some kinds of parasite will infect different hosts in various stages of growth. For instance, Anisakis, nematodes that are the cause of anisakiasis, first incubate in water, then are preyed upon by crustaceans such as krill. They avoid digestion and survive in their bodies until becoming a stage three larva. Next, the crustacean is eaten by a fish higher up on the food chain, and the Anisakis continues to mature inside the fish's body. Then the fish is eaten by a whale, and the Anisakis can pass stage four as a larva and become an adult within the whale's intestine. The eggs laid by the adult mix in with the whale's excretions and discharge into the water.

This is the life cycle of Anisakis. In its case, the crustacean would be the "first intermediate host," the fish the "second intermediate host," and the whale the "final host." A final host is a parasite's final destination. If it can't infect its final host, the parasite can't reproduce.

"...So, back on topic. How many infected by Toxoplasma do you suppose there are worldwide?", Sanagi questioned.

"You say they can infect most warm-blooded animals, so I'm sure it's a pretty big number. A few hundred million people?"

"Over one-third of the population," Sanagi said readily. "A few billion people."

Kousaka's eyes went wide. "That many?"

"If we restrict it just to Japan today, I guess the ratio might be a little lower. Maybe ten or twenty percent at best."

"Either way, that's still a lot. ...But on the other hand, that's proof that Toxoplasma is harmless to humans, right? If that weren't the case, I'd think there would be a big panic by now."

"Yeah. It's no problem if it infects healthy people. And so far, it's been deemed harmless to anyone but those with immunodeficiency and pregnant women. But lately, there's been talk about the possibility of them altering people's actions and personalities."

Sanagi poked at her forehead.

"There's been interesting research about the effects of giving Toxoplasma gondii to a host. Male rats who are infected with these protozoa stop fearing cats, who should be their predators. Apparently the Toxoplasma controls the rat, using it as an intermediate host, to make it easier to be eaten by its final host, the cat."

"It controls the host?" Kousaka's voice cracked in horror. Wasn't that just like Heinlein's book The Puppet Masters?

"When they dissected the infected rat, the area around the cerebral limbic system had a huge number of cysts. And when they analyzed the DNA of Toxoplasma gondii, they found the presence of genes relating to the creation of dopamine. I don't know the exact mechanisms, but it seems like Toxoplasma can control a host for the convenience of its own reproduction. In fact, parasites controlling their hosts is a common thing. Dicrocoelium and Leucochloridium are famous examples. Both are known to cause suicide or starvation in intermediate hosts."

Kousaka thought a little, then spoke. "So you mean something similar could happen in human brains infected by Toxoplasma?"

"That's right. Recent research shows that a man infected with Toxoplasma gondii shows a more favorite reaction to the smell of cats than a man who isn't infected. However, it seems the opposite was shown in women."

"That's strange. There's a gender difference in the effects of parasites?"

"I haven't heard much about it with other parasites, but it's a trend I see with Toxoplasma research. There are results showing that infection by Toxoplasma gondii causes men to become antisocial and be disliked by women, while women become social and are liked by men. There was also a report that for women, there were 1.5% more who attempted suicide among the infected than the uninfected."

"So Toxoplasma might induce suicide in women?" Kousaka shuddered. "And over a third of the world's population is infected with a parasite like that?"

"It's just a possibility. It's not proven."

"...Even so, it makes me shiver," Kousaka said with a sour look. "They say Pasteur became a germaphobe because of his studies in microbiology, so I feel like the more I learn about things I can't see with the naked eye, the harder it'll be to live in this world."

"I've got plenty more spine-chilling stories where that came from. Want to hear?"

Kousaka shook his head. "No, let's change the subject. Sanagi, do you have any interests besides parasitology?"

"Hmm... It's a secret." Sanagi put her finger to her lips mischievously.

"Is it a hobby you can't tell people about?"

"It's a girly hobby."

"Normally you would be public about the girly hobby and hide the parasite-loving."

"Standards for embarrassment will vary," Sanagi retorted with displeasure. "You tell me, Mr. Kousaka. What captivated you about making viruses?"

Kousaka told the story of acquiring an interest in malware. How a text message about the end of the world saved him somewhat. How he wondered if he could make something similar himself. How he found once he started that he was unexpectedly suited for it, and it even ended up being what he lived for.

"I think I kind of understand feeling saved by a message about the end of the world," Sanagi agreed. "By the way, what kind of virus were you making?"

"Do you know about the first computer virus recognized in Japan, Sanagi?"

"Nah."

"Japan's first domestic virus was developed in 1989. It was a sort of playful virus called Japanese Christmas that just displayed a festive message on computers on December 25th. Similarly, the malware I made will activate on Christmas Eve. Though I think the damage it'll bring about will be a little more serious."

Sanagi moved her chin just a few millimeters, urging him to continue.

"Strictly speaking, what I made is a worm that isolates people," Kousaka explained, trying to break it down. "Infected smartphones can't do any communication from the evening of Christmas Eve to Christmas night. I figured it would mess up couples trying to meet up all over Japan. ...Funny, isn't it?"

But Sanagi didn't laugh.

The moment she heard Kousaka's words, her eyes widened and she became still, as if struck by a bolt of lightning.

"What's wrong?", Kousaka asked. Sanagi's eyes remained fixed on his throat, and she didn't reply. And perhaps her eyes didn't see anything.

Sanagi didn't move for a while, silently pondering. As if she'd found a crack in the world, she kept staring at the same point in space. If you listened closely, maybe you could hear the gears rapidly spinning in her head.

Maybe something in my words disturbed Sanagi, Kousaka realized. But he couldn't identify anything that had such power in what he'd said.

In the end, Sanagi never explained why she suddenly went silent, and awkwardly changed the subject. But even while chatting about different subjects, it seemed her attention was still focused on "something" from earlier.

It only figured that she would be disturbed. Because the malware Kousaka created, as it happened, was just so similar to something else she knew of.

*

It was his weekly shopping day. Holding shopping bags in both hands, Kousaka walked down the road lit by streetlights. Water that had faintly pooled in places on the road shone darkly. The air was clear, and you could see even small stars with the naked eye.

He saw a middle-aged man sitting on a roadside bench encircled by trees. When the man saw Kousaka, he put his can of coffee on the bench and stood up.

"Hey," Izumi said, raising a hand. "Looks heavy. Need a hand?"

"I'm fine." Kousaka turned him down. "...Checking on my progress?"

"Well, more or less."

Izumi was in his usual get-up, a drab Chester coat over a suit. Did he not have any other coats? Or maybe he decided to always wear this when he met Kousaka? Or maybe it was simply that he didn't care at all about clothes.

Izumi sat back on the bench and glanced at Kousaka's shopping bags. "I've been wondering, what does a clean freak eat?"

"Cereal, nutritional foodstuff, tofu, canned goods, frozen vegetables..." Kousaka listed off the contents of his bags. "There's a lot I won't eat, sure, but I'm not particularly limited. And I don't generally eat a lot."

"Meat? Sashimi? Raw vegetables?"

"I hate oily things, so I can't eat meat. Sashimi is a definite no. I can eat raw vegetables if I clean them well and cook them myself. Though I don't think about wanting to eat them because I like to."

"Alcohol?"

"I'll drink only whiskey, if I'm told to."

But that only applies to medicine-esque whiskey like Laphroaig and Bowmore, Kousaka thought to himself.

"Well, that's good," Izumi nodded, finding it plausible. "There's lots of people who can't drink whiskey and aren't clean freaks. In that sense, you're pretty lucky."

Kousaka sat down beside Izumi and put his bags on the ground. The cans in the bag clinked up against each other. After pulling the face mask dampened by his breathing down to his chin, he spoke.

"The reason Hijiri Sanagi hasn't been attending school is scopophobia."

After a few seconds, Izumi asked, "Did you hear that from her mouth?"

"Yeah. Her headphones seem to be for easing her symptoms."

"...Hard to believe," Izumi said doubtfully. "Did Hijiri Sanagi really say that? This isn't just some guess of yours, is it?"

"Have you not heard anything from her?", Kousaka inquired.

"She won't tell me anything about herself. It's a total mystery."

I see, Kousaka thought to himself. From Izumi's face just now, he could surely conclude that there was some level of communication between Izumi and Sanagi.

"She happened to have an episode of scopophobia and called me for help. If it hadn't been for that, I probably wouldn't have been able to find out for quite some time."

"She called for help?", Izumi repeated, as if struck off-guard. "This is shaping up to be a hell of a surprise. Can't tell what'll happen with you. And I figured you had the least promise of anybody I'd hired to date..."

"She probably had no one else to depend on then but me. I was just lucky."

"No, I don't think that's it. You're the first person to find out why Hijiri Sanagi hasn't been going to school. Until now, no matter how weak-hearted she's been, she's never revealed her scopophobia to anyone except relatives. In other words, she trusts you as much as a relative."

I'm as happy as can be if that's true, Kousaka thought. But he couldn't take Izumi at face value. Maybe he was fabricating this as a way of flattering Kousaka. It wasn't too strange to think that all the people hired before him had been taken by this tactic.

Izumi took an envelope out of his coat's inside pocket and handed it to Kousaka.

"Your payment. But it's only half. Whether I'll pay you the other half depends on your actions after this."

Since it was half, that meant it would match what he'd paid Sanagi. Relieved his investment came back to him, Kousaka put the envelope in his pocket.

"...So, what should I be doing next?"

Izumi didn't have an immediate answer, leaning back in the bench and looking at the sky. Kousaka also leaned back and looked upward. He thought maybe it had started to snow, but that didn't seem to be the case. Izumi was pondering something. It almost looked like he was searching the countless stars to find an answer.

Izumi took a drink from the can of coffee beside him, breathed, and answered the question.

"You don't have to do anything."

Kousaka turned to Izumi with wide eyes. "So does that mean my job is -"

"Hey, I didn't say that. Your job isn't over just yet. By "you don't have to do anything," I mean "keep things as they are." Keep being a reliable friend to her like you have. If you do that... maybe something interesting'll happen."

"Interesting?"

Izumi ignored his question.

"That's all from me. I'll contact you again later."

With that blunt remark, Izumi got up from the bench. It looked like he was going to just leave, but he suddenly stopped and turned.

"Forgot to say something important. A thing I should warn you about."

"What is it?"

"Whatever happens next, don't cross the line with Hijiri Sanagi. Even if that's what she wants from you. I doubt I need to worry about a clean freak like you, but I'll put it out there as a "just in case," against all odds. Keep it platonic, like Signal and Signaless."

Kousaka looked at Izumi dumbfounded. Then after a delay, he furrowed his brow.

"What are you talking about? You do know how much of an age difference there is between us?"

"Just tell me "yes." I'm not saying this out of concern for Hijiri Sanagi, I'm saying it for you. If you ignore that warning, you'll be the one in the most trouble. Up to you if you'll believe that or not."

Kousaka sighed. "You have no reason to worry. I won't even be able to hold her hand."

"Right. I'll be praying it stays that way."

With that, Izumi vanished into the cold darkness.

*

Kousaka was summoned by Sanagi via a phone call. It wasn't a desperate call like last time, but there was a sense of her having some business for him.

"There's something I want to try. Come to the library right away."

With only those words, Sanagi hung up. Kousaka hesitated for a while, but soon gave up, changed clothes, donned his gloves and face mask, and prepared to go out. But just before he left the room, he resolutely took off the mask and threw it in the trash can. He didn't know why himself, but he thought it would be better that way.

Sanagi was waiting for him on the stairs by the front door of the library. As usual, she was dressed in a way that seemed to leave her legs cold, and she was in fact shivering slightly, but she seemed to consider this shaking natural. When she saw Kousaka, Sanagi took off her headphones and raised her hand a little.

"What did you want to try?", Kousaka asked.

"I can't answer that right now. I'll tell you in a little bit."

Sanagi stood up. They walked together.

On the walk to the apartment, Kousaka kept stealing glances at Sanagi's face. He hadn't thought anything of it before, but after Izumi's unfair suspicions, he was acutely aware of Sanagi today.

Kousaka tried asking himself: Could I see this parasite-loving, scopophobic girl in a romantic way? Soon, an answer came back. "Not a chance." Indeed, it was the undeniable truth that I felt no special feelings toward Hijiri Sanagi. But I did have a perfectly natural fondness for her simply as someone dealing with similar worries, which was out somewhere very far from romance.

Kousaka laughed off his unease - idiotic. She was still a kid in her teens. Izumi couldn't have been speaking seriously, either. Just making extra, extra sure, no doubt.

Suddenly, he noticed Sanagi was looking right at him. He was worried that the things on his mind were showing on his face, but it didn't seem that was it.

"Hey, Mr. Kousaka. If I asked you to stroke my head again right now, what would you do?"

For a moment, Kousaka couldn't respond to the unexpected question.

"You want me to do that?"

"Just hypothetically. Could you? Couldn't you?"

Kousaka mentally evaluated the hypothesis.

"If I tried, I don't think it would be impossible."

"Yeah, right?"

"...And?"

"When I'm walking with you like this, I'm fine without headphones."

Now that she mentioned it, she'd taken her headphones off at some point and put them in her bag.

"It looks like when you're here, my scopophobia gets a little better. Maybe it could be relief from having someone who properly understands my symptoms around. What about you, Mr. Kousaka?"

Kousaka suddenly put his hand to his mouth. And it hit him. That was more or less why he'd removed his face mask before leaving the room. There must have been a sense of relief knowing he was meeting up with Sanagi, so he felt more at ease than usual.

"Yeah, I guess when I'm with you, my phobia is a little less intense, too."

"I knew it," Sanagi said proudly. "I don't really get how, but we can't not make use of this."

"Make use of it for what?"

"Isn't that obvious? Training ourselves to get used to the outside world. A three-legged race of rehabilitation, so we can walk around outside without gloves or headphones."

"...I see. That's not a bad idea," Kousaka affirmed.

"And so I did some thinking..."

Sanagi quickly began to outline her plan.

December 17th, Saturday.

Thinking about it, it was the first time Sanagi came to the room before noon.

When they met, Sanagi gave Kousaka a Shinkansen ticket. He'd heard they were going to go further out than usual, but thought it would still be within the prefecture at best, so this made him nervous.

He was about to pay her for the ticket, but Sanagi plainly denied him.

"This is a present from me, so I don't need money. In exchange, you can't complain no matter what our destination is."

"Got it," Kousaka acknowledged. Then he quietly appended "As long as it's not too dirty."

The two left for their destination. They kept gloves and headphones in their bags in case they were needed, but they were meant to be last resorts. Unless something came up, they didn't intend to take them out.

Kousaka had hardly any memory of the trip. He was desperately trying to not think about anything, leaving him no time to relish the scenery or have a conversation. Sanagi was much the same way, hiding her face and fidgeting the whole time they rode the train.

Indeed, their symptoms were much weaker than usual. But it was akin to your body temperature dropping from 104 to 102, so while it was quantifiably "better," it didn't change that they were serious disorders.

When they got off at Tokyo Station and switched to the Yamanote Line, Kousaka's anxiety peaked. It was terribly crowded in the train, and every time it jolted, he bumped into nearby passengers, which repulsed him like insects were crawling all over his body. Just breathing felt like it contaminated him with the air others were breathing.

His stomach ached, and he felt very nauseous. Something sour was welling up in his throat. He was unsteady on his feet, and could well fall over if he wasn't careful.

But Sanagi was beside him. She held the cuff of Kousaka's coat, desperately resisting her phobia and grinding her teeth. When he became conscious of Sanagi's presence, his stomach pain and nausea slowly retreated. I'm the only one Sanagi has to rely on at this moment. What'll happen if I can't hold myself together?, Kousaka thought to encourage himself.

"Are you okay?", Kousaka asked quietly. "Can you keep going?"

"Yeah. I'm fine," Sanagi answered hoarsely.

"If you can't bear it, tell me right away."

"It's you who looks terrible," Sanagi laughed, acting tough. "If you can't bear it, tell me right away."

"I'll do that." Kousaka laughed, too.

The ride lasted less than 20 minutes, but to paraphrase Einstein, it was twenty minutes of putting your hand on a hot stove. When they got off the train, Kousaka felt exhausted like he'd been trapped in there for two or three hours.

After getting off at Meguro Station and walking west for about 15 minutes, Sanagi stopped.

"We've arrived."

Kousaka looked up. In front of Sanagi was a compact, six-story building. On the building were the words "Meguro Parasitological Museum."

Parasitological Museum?

"Doesn't look like much of a place for me," Kousaka meekly protested.

"You promised you wouldn't complain no matter where it was, right?" Sanagi shifted her head and smiled.

He didn't have the energy left to defy her.

Following after Sanagi, Kousaka entered the museum. In an area resembling a small waiting room was displayed information about and samples of parasites. The two of them started at one end and looked at them in order. There were rows of specimen bottles lined up in glass cases containing various types of parasites, as well as some of the creatures and even organs the parasites inhabited.

Until he actually saw them, Kousaka was concerned that just looking at samples of parasites might make him sick enough to faint. But the parasites in the medicine-soaked bottles looked more like abstract sculptures than insects, so they struck him as surprisingly clean.

Some of the parasites had appearances resembling noodles or vegetables. Hookworms and beef tapeworms were like curly kishimen noodles, roundworms like tangled bean sprouts, Gyrocotylidea like cloud ear mushrooms. Of course, there were a number of grotesque samples that made him want to look away as well: a field mouse with echinococcosis that had a massive tumor on its stomach, the head of a green sea turtle infected with Ozobranchus branchiatus. Kousaka's face reflexively twitched when he saw these, but Sanagi admired them without issue. ƒ𝘳𝗲𝑒𝒘𝐞𝚋𝚗𝐨v𝘦l.𝒄o𝐦

Besides Kousaka and Sanagi, there were five other groups of two visiting the museum, and four of the groups were couples. Kousaka had a hard time understanding why that many people would go here for a date. Some of the couples were making a clamor like they came here wanting to see something scary, but others were casually discussing the exhibits and throwing technical terms around.

"Mr. Kousaka, look."

Sanagi spoke, having been looking at the displays silently until then. She was looking at a specimen bottle with a sticker labeled "Diplozoon paradoxum." The caption said "It looks like a single butterfly at a glance, but this special parasite is two worms who met as larvae and became one" - more or less coinciding with what Sanagi had told Kousaka. The founder of Meguro Parasitological Museum, Satoru Kamegai, studied this parasite as his life's work, so it was also like a logo for the museum.

Kousaka looked through the magnifying glass installed in front of the bottle.

"Well?", Sanagi asked from beside him.

"...They're butterflies."

Indeed, they looked like butterflies. White-ish butterflies with small rear wings. They had nearly the same shape as the keychain Sanagi had.

Kousaka squatted in front of the glass case and watched the D. paradoxum samples for some time. Somehow, Kousaka felt like these symbolically-shaped parasites were very nostalgic.

The panel on the second floor featured well-known parasites like Toxoplasma gondii and Echinococcus, and there was also an explanation of Spirometra erinaceieuropaei. According to the explanation, when S. erinaceieuropaei infects a human, it can cause a parasitic disease called sparganosis. The disease's Japanese name contains the term "orphan worm," referring to insects whose larval form has been identified, but not their adult form.

"Strictly speaking, the use of "orphan worm" isn't accurate," Sanagi added from the sidelines. "When S. erinaceieuropaei was first discovered, their adult form was unknown, so they were treated as orphan worms for over thirty years. So the disease's name incorporated this term, and even though the adult form has been found now, it keeps that name for customary reasons."

As usual, she gets so talkative when it comes to parasites, Kousaka thought with a smirk.

Sanagi pointed to the right side of the glass case.

"On the other hand, the adult form of these Sparganum proliferum has gone unfound for over a century, so they're real-deal orphan worms. When they infect humans, they repeatedly split up inside the body to multiply, and invade and destroy all sorts of systems, including the brain. Ultimately, the infected person is completely full of them and dies. There's currently no established treatment, so the fatality rate is 100%. Medicine doesn't work, and they're too numerous to do anything surgical."

Kousaka gulped. "There exist parasites that dangerous?"

"Yeah. Of course, there's only ever been a dozen or so reports worldwide of them infecting humans."

Then the two of them looked at the samples in silence for a while.

"Hey, Sanagi, I just thought something," Kousaka said, still looking at the Sparganum proliferum. "Why would they kill humans? From what I'm hearing, what these things do is like a double suicide. If they kill the host they're inhabiting, they'll go with it, right? Why would they sink the island they live on?"

Sanagi looked toward Kousaka as if to say "that's a good question."

"Parasites aren't always necessarily able to infect who they want. Sometimes they end up lost in a non-definitive host - a host that's not intermediate nor final, and can't even become a standby host. To a parasite, infecting a non-definitive host means forever losing the chance to inhabit their final host. When this happens, most parasites will just die off, but some will stubbornly resist, and in trying to reach a definitive host, move around organs and systems as larvae. In some cases, this can cause symptoms of serious illness. It's a class of disease called larva migrans. When Gnathostoma, which infect freshwater fish, infect a human instead, they can go astray in their body for over a decade."

"It's just trying to get away from the body it wandered into by mistake?"

"I guess that's pretty much it. Even that terrifying Sparganum proliferum should be docile when it infects a definitive host. Because you're right, if they kill their final host, they'll just be going down with it."

Kousaka nodded. Come to think of it, he'd heard Echinococcus was spread from foxes to humans, and was harmless as long as it inhabited foxes.

Sanagi went on talking eloquently. "That said, it's not like parasites will never harm their final hosts, either. For example, beef tapeworms are parasites that infect humans as final hosts, but when these larvae invade the brain and spinal cord, they cause cysticercosis, a very lethal disease to us. And -"

Suddenly, Sanagi's mouth shut. She'd realized the visitors around them had gone silent and were listening to her talk. While some were looking at her like some curious creature, there were others who were genuinely impressed. Sanagi looked around, realized she'd unintentionally been drawing attention to herself, and hurriedly hid behind Kousaka's back.

"...We should get going," she said in a wispy voice.

"Yeah," Kousaka affirmed.

If Sanagi had finished her explanation of cysticercosis that day, the outcome of a later incident might have ended up a little different.

When people ingest beef tapeworm eggs, the eggs incubate in their organs, and become larvae called bladder worms. Bladder worms move through the intestine, and create cysts there. When these cysts form in places like the brain and spinal cord, it causes cysticercosis, but in truth, the symptoms of it almost never appear while the bladder worms are alive.

The problems arise after the bladder worms die. The death of bladder worms in the central nervous system causes a strong tissue reaction. It causes local inflammation and glioma around the cysts, which can result in nerve damage and epileptic fits. At this stage, the death rate from cysticercosis can be as high as 50 percent.

There was an important reason why none other than Kousaka should have had this knowledge. For a layman of parasites like himself, it may have been downright impossible for him to ignore preconceptions and put together the knowledge to arrive at the truth.

*

Compared to the trip there, the return trip was much easier. After a short rest at a café to get a light meal, they headed home. The two of them chatted idly their entire time on the Shinkansen.

"Come to think of it, I happened to hear once that parasites can cure allergies. Is that true?"

"There have been experimental results showing that. Not only allergies, but they seemed to be effective against autoimmune disorders like ulcerative colitis and Crohn's disease. But naturally, healthiness can't be guaranteed, so using them domestically as cures is probably a long way off."

Kousaka twisted his neck. "How does that work, exactly? Normally, I'd expect the introduction of something foreign like a parasite to cause serious allergies."

"That definitely happens too, of course. But..." Sanagi was silent for a few seconds, like she was unpacking a compressed memory. "In some ways, human immune systems were formed with the assumption of parasites existing. There would be a panic if we found parasites coming out of our bodies now, but just a few decades ago, it was normal to have various parasites. If our immune systems attacked them one by one, the human body would be a constant battlefield, and be torn to shreds in the blink of an eye. So our bodies are built to choose a path of coexistence with relatively-harmless intruders."

"Peaceful coexistence, huh?"

"Right. It has to do with the cells that suppress immunity, called regulatory T cells - depending on the person, there may not be enough of these cells, and immunotolerance won't kick in. As a result, the immune system may excessively attack foreign substances, or even become hostile to your own cells and systems. Broadly speaking, this is what causes allergies and autoimmune diseases. Thus, making the immunosuppressive system work is linked to bettering immunity-related diseases. But apparently, these regulatory T cells are brought about by the existence of "parasites approved by the host." So in essence, the absence of parasites, an extremely clean situation, results in an increase in modern day allergies and autoimmune diseases."

Kousaka thought it over for a bit. "In other words, parasites can cure allergies by cleverly weakening the immune system's defenses?"

"If you break it down, I think that's about correct."

Kind of reminds me of Freud's Eros and Thanatos, Kousaka thought. That was all about how energy which was outward-facing could be turned inward and become self-destructive.

"Still, it's kind of shocking to hear that the human body is "built on the assumption of parasites.""

"Is it really? Isn't intestinal bacteria a clear example of that?"

Kousaka could understand that. Now that she mentioned it, it was true.

While walking down a corridor on the second floor of a train station they got off at to change trains, Kousaka casually looked out the window and over the street in front of the station. The trees were decorated with Christmas lights, and the street was colored by fantastical orange light. As he looked over toward Sanagi, he saw her also staring at the decorations. Her eyes had a complex mix of scorn and envy.

A few dozen minutes after changing to a private railway, they finally saw some familiar streets. They left the station and took in the fresh air again. The night sky was pretty and clear, and you could easily see the half-waned moon.

"Seems we made it back okay," Sanagi said emotionally.

"Somehow," Kousaka said. "That was pretty rough for our first training."

While walking through the silent residential district, Sanagi came to a sudden halt. She was looking at a children's park. A cramped one that didn't seem suitable for catch or tag or anything. Sanagi stepped toward it without hesitation. Kousaka followed.

Seemingly unused for a long while, there was a staggering amount of snow piled up in the park. Each step had their feet sinking into the snow up to the ankles. It was snow that solidified easily, so by stomping on the snow piles in your path to make footholds as you went, you could avoid getting snow in your shoes.

Reaching a bluish-green jungle gym, Sanagi went up without hesitation. She sat at the top, went "it's cold, it's cold" as she warmed her hands with her breath, then looked down at Kousaka below and grinned proudly.

Kousaka timidly touched the jungle gym. And brushing away snow to avoid slipping off, he carefully climbed up to sit next to Sanagi.

He hadn't climbed a jungle gym since grade school. The two were silent for a while, enjoying the nostalgic yet fresh sensation. Just by having a view a mere two or three meters higher, the world looked somehow different from usual. The snow in the park absorbed the moonlight and glowed a pale white.

After some time, Sanagi broke the silence.

"Mr. Kousaka, do you remember the Diplozoon paradoxum I told you about earlier?"

"Of course I remember. Looks like a butterfly, fateful love at first sight, lifelong copulation, love is blind, two peas in a pod?"

"Fantastic." Sanagi brought her hands together and smiled. Then she continued. "Hey, Mr. Kousaka, have you ever thought about this?"

Maybe my whole life, I won't have anyone to call my partner.

Maybe I'll die without ever being in love with someone.

Maybe when I die, there'll be no one to shed tears for me.

"I'm not D. paradoxum, so sometimes before I fall asleep, I think about things like that," Sanagi said casually without any emotion. "Do you understand those feelings, Mr. Kousaka?"

Kousaka nodded deeply. "I'm always thinking about that kind of thing. When I'm walking around and see a perfectly happy-looking married couple, I really think about it. "That's probably something I'll never have all my life." It makes me unbearably sad to think so." Then he took a breath and made an addition. "But I don't think you need to think about things like that, Sanagi. You're still much younger than me, and clever, and honestly, good-looking. You have plenty of things to make up for your faults. I'm sure there's no reason to get pessimistic so soon."

Sanagi slowly shook her head. "Mr. Kousaka, you can only say that because you don't know me very well."

"You might be right. But it's a mistake to think you're the one who knows yourself best. There are things you overlook about yourself. Sometimes, what other people see might be closer to the truth."

"...I guess so. It'd be nice if that were true."

Sanagi's eyes narrowed sadly, and she opened her mouth to say something, but rethought it and pursed her lips. Then she slowly sat up.

"We should go home soon. I'm getting pretty chilly."

"Let's do that." Kousaka got up, too.

After leaving the park, the two were completely silent. Ultimately, it came time to part without them having said another word to each other. Kousaka began - "Well," - to say goodbye, but Sanagi interrupted him.

"Um, when you do something, I think it's best to have a clear goal in mind."

It took Kousaka about five seconds to realize she was talking about trying to conquer the obstacles posed by their compulsions.

"So, how does this sound? By Christmas Eve, I'll be able to walk around town without people's gazes bothering me. Mr. Kousaka, you'll be able to hold hands with someone without dirtiness bothering you. If we achieve those goals, then on Christmas Eve, we'll hold hands and walk by the Christmas lights outside the station, then have a modest celebration."

"Sounds like fun."

"Then let's promise."

With that, Sanagi turned and quickly left.

After getting home, without any particular intention, Kousaka looked up Meguro Parasitological Museum. And a surprising fact came to light. It seemed Meguro Parasitological Museum was a famous date destination in the area. That's why there had been so many couples.