One of the smartest ways to win in the game Plague Inc is to make your virus have mild acute symptoms that are easy to dismiss or confuse as other things, and then really fucked up chronic symptoms later on. Symptom severity is not the same as underlying disease severity. The immune system doesn't always enter the panic mode entailed by a fever, in some cases because the virus has outsmarted it. HIV is a virus that starts as a cold, and then does all its damage silently. Judging a virus by its acute symptoms is one way to fuck around and find out