It may interest you that a virus which kills its host won't spread as well as viruses which co-exist. This is basic from epidemics transmission rate. The longer an infected host keeps spreading the infection on, the more individuals will be infected. If the host is killed it will no longer spread the infection and therefore the infection may infect fewer hosts and this reduction in transmission rate alone may prevent a major outbreak.

https://en.wikipedia.org/wiki/Mathematical_modelling_of_infe...