If you were an infection that wanted to survive and spread as far as possible, you wouldn't want to kill your host at all.

In fact, if your host lives for as long as possible without suffering any negative effects from your freeloading, is preferred by mates and most loved by peers, etc., that's when you spread the furthest.

Seeing it as a nefarious plot to kill as many as possible (by prolonging incubation periods, etc.) is humans thinking in terms of how their species behaved in the past. In fact there is only maladjustment.