COTE : Parasite in Love.

Chapter 9: Winter Wormderland I

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."

Ayanokouji 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..." Hiyori 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."

Ayanokouji 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, Ayanokouji 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?"

Ayanokouji 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, Ayanokouji 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 Hiyori, 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," Hiyori said emotionally.

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

While walking through the silent residential district, Hiyori 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. Hiyori stepped toward it without hesitation. Ayanokouji followed.