Every once in a while, most people get nasty feelings that they don’t want other people know, like jealousy. Say there are specific biological pathways and chemicals involved in causing such feelings and say, that a superb mutational capacity of some viruses allows them to interact and invade these pathways/chemicals. In other words, only the people who have these feelings get sick.

That might annoy many people…or maybe some virus might have done it before and people just didn’t notice it.