People are NOT the same thing --- Polio and smallpox vaccines were not given to every single person for dozens of years annually or every few years. Dogs can have ten generation pedigrees where every single dog in that pedigree recieved the exact same vaccinations. Not only does this mean that the dog is breeding a weaker immune system into it's line, but that dog is carrying on a higher chance of vaccine sensitivities. Smallpox was not something that was vaccinated into the entire world population for years and years and years. Vaccinosis is extremely rare in people compared to dogs because people are not vaccinated as often and not with nearly as dangerous protocols. People are not able to start breeding at a young age and create litters that pass on the same genetic predispositions every year. Smallpox and polio compared to parvo virus is not the same thing at all. Parvo is one of the more common vaccinosis reactions, it has been known to last on sterilized surfaces for up to 7 years, etc. There is no way that every canine and feline disease could be wiped out as easily as polio and smallpox were.