If you were a neat new virus, would you want to be deadly, or harmless?
Many evolutionists agree, viruses tend to mutate towards harmlessness. The thought goes like this: deadly viruses kill their hosts before they can spread. The less deadly the virus, the more likely the host is to survive at least until the virus is spread to a new host. So viruses tend overall to mutate away from deadly.
This should be good news to anyone fearing a global kill off of all humans by a new supervirus or superbacteria; any bug that wants to live most probably must sacrifice its own virulence for the sake of its own infectiousness.