> If a a virus is so deadly, everything it touches dies soon, it would not spread quickly but die out. If it is very contagious .. but very, very slow incubation time, so it infects the whole world, before becoming a deadly disease ..

This is a made up equilibrium that actually does not need to exist in nature.

Viruses and bacteria can in fact be both extremely, extremely contagious and extremely, extremely lethal.

> If a a virus is so deadly, everything it touches dies soon,

Trivially: you actually can have a virus that kills everything it touches not soon. Nothing in biology or chemistry or physics prevents it.