There are very few viruses that can get past innate and cellular immunity and cause infection from “a half dozen virus “cells”.”

In general, I can only think of a few that are known to be that infectious.

Also, I’m general, viruses do not want to kill their host. Usually the strains that mutate and survive are strains that remain highly infectious but not highly lethal.

Most complications from infection come from the body’s overactive response, things like “cytokine storm.”