A little refresher on polio vaccines: There are two, the original injectable that uses killed virus (Jonas Salk’s original vaccine) and the oral drop version that contains a weakened live virus (Albert Sabin’s formula). The oral vaccine was the first one used in the international eradication campaign, because it is inexpensive to make and can be administered by anyone. It is still used in the developing world, though industrialized countries have gone back to the original injectable.
For all its benefits, the oral version has a known issue, a combination of bug and feature. Once it is given, the vaccine virus replicates in the gut. The feature part is that, when the vaccine virus passes out of the body, it can spread through the environment of places with poor sanitation, conferring a kind of passive secondary immunization on others nearby. The bug part is that, in the few weeks it is replicating, the vaccine virus mutates, and sometimes mutates back past the artificial weakening to the original disease-causing form.British health authorities are aware of a man of about 30 who has been shedding live polio virus in his waste since he was a small child. It's a rare problem, requiring bad luck in the mutation of the live virus in the man's childhood oral vaccine combined with a immune system disorder just bad enough to let the polio virus live and not bad enough to kill him quickly. Occasional short-term vaccine-induced polio outbreaks have been known to happen on several dozen occasions since the polio-eradication program began in the 1950s. They're a terrible problem, not only because any unvaccinated or immuno-compromised locals are at risk of an awful disease, but because vaccination programs are controversial enough in the some areas of the world without having to make suspicious communities even more suspicious of outsiders' plot to sicken and kill them with mysterious medical technology.