Memes (ideas/beliefs) behaving like viruses is the subject of lots of prior material. Delusions fit in this category just like things like fashion/religion/culture/language do. They don't directly spread physically, but by others being exposed to the concept whether via social media, in-person contact, books, whatever.

I don't think you can extrapolate too much about physical/chemical mental illness aside from some people being more receptive to certain ideas/delusions than others (maybe from background/upbringing, or just personality).