I once statted up a viral "species" who was contagious via similar vectors to AIDS. The Toxic Attack that their Infectious Attack used had Blood Agent for infecting the target, but on the other side of things was an Either/Or limitation of Reverse Blood Agent
or Requires Sexual Intimacy, -10%. For those curious, the virus turned people into creatures that can only eat live prey and have
Horror Hunger for humans in particular. They still looked human save for a couple of minor features. However, their existence was well known, and they were pretty much restricted to ghettos. What made things worse is that some people were carriers, who didn't change much (most wouldn't know they were infected), but can spread the virus.