Smallpox? Are you sure it wasn't polio?
Sewage is routinely tested for polio. Smallpox was spread by aerosols and contact with contaminated items, so it would have never been tested for in sewage. Smallpox was strictly a human disease, so when it was wiped out, it truly was wiped out. There are other pox diseases that people can catch, but they are not smallpox.
It was polio.
Sorry. 14 hour days and not enough coffee lead to bad FREEPING