True but cowpox vs smallpox is a trivial counterexample but it certainly isn't complete protection given the many strands of the common cold and flu. To make a

True but cowpox vs smallpox is a trivial counterexample but it certainly isn't complete protection given the many strands of the common cold and flu. To make a complete hash out of the actual details of biology for the sake of explanation we could say that cowpox shared the same immune signature as smallpox while being less deadly resulting in any smallpox infection attempt being nipped in the bud by the immune system. In that case the question is "Would there be more deadly alias viruses counterparts out there and how does the relative risk profile out?" There is also a matter of niche and the fact we would strongly prefer that mostly harmless ones fill the niche rather than deadly ones but we still sterilize instruments because we rationally don't want to take the chance that our "harmless" microbes might mutate to something more deadly on us, let alone the location dependent danger like ecoli in the digestive tract is fine but not in your bloodstream.