When a virus has been associated with its host for a very long time, it mutates to a form that grows well without killing its host, which would be a pointless outcome from an evolutionary standpoint.
Je-li virus se svým hostitelem ve styku po velmi dlouhou dobu, mutuje do podoby, která dobře roste, aniž by svého hostitele usmrtila, což by z evolučního hlediska byl nesmyslný výsledek.