Parasite in Love by Sugaru Miaki

Translated from Japanese Completed Psychological Romance

The story is about a man whose compulsive tendencies make it impossible for him to keep a job, and a young girl who skips school and loves insects. The two meet, come to support each other in reintegrating into society, and fall in love. There’s a problem, however: namely, the parasites in their heads.

The colorful extravagance of some birds,
the deliberately excessive horns, manes, and tusks of some mammals,
the complex courting behavior of many animals,
the very existence of sex itself -
and by extension the love songs on the radio,
and all the love-addled poems ever written -
all likely evolved because of parasites,
because all organisms must run as fast as they can
just to stay in place.

- An Epidemic of Absence
Moises Velasquez-Manoff

1: Parasite in Love