Three Days of Happiness is a novel by Sugaru Miaki, also known as Fafoo. It’s a rewritten version of the story originally posted to 2ch.

If you haven’t read the original story, the premise revolves around a shop where one can sell away the remaining years of their life, and the consequences of doing so.

Also, I Say A Little Prayer is a short related story which was posted along with the novel’s release. It spoils some things about the main story, but nothing major.

(Source: Novel Updates)

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.

A person who has the power to control people's bodies uses it to make them commit suicide, and one day encounters a special target.