>>132921945Author originally wrote it around 2005. I don't know about Japan, but I know there were a lot of AIDS awareness campaigns around where I am, at the time.
He could have figured "the disease fits what I need of it, let's use it."
Ultimately, there's just not many diseases that can spread to a whole family like that on accident, and kill the whole family over the course of several years, and not all at once.