[onlyrealcuzzo]

Look for counter examples. Have we seen this in HIV? Smallpox? It's a plausibility. It's far from a guarantee.

And the opposite, /could/ happen, too. The virus could mutate to have a longer incubation period, or be more deadly, or both.

[rimliu]

I have seen a statemen somewhere tha for viruses it goes "transimissability, mortality, stealthiness: pick two". Not sure how true it is, but looks plausible.

[Metacelsus]

At least for natural viruses. Engineered ones could be extremely terrifying.