polio

one of the problems of oral polio vaccine is that it is an attenuated "live" vaccine: a weakened form of the infection. Alas, in one out of a million cases, it will mutate back to the severe form.

this is not a major problem because with universal vaccination, the mutated virus does not spread, because every one is immune.

But that's not what happens where vaccination rates are low

that is what happened here.

But note: the numbers are low:

 In 2019, 358 cVDPV2 cases were reported, representing a fourfold increase over the 71 cases reported in 2018 and more than tripling the number of countries with outbreaks, from five (2) to 16. As of August 2, a total of 236 cVDPV2 cases in 17 countries have been reported in 2020. Among 33 cVDPV outbreaks reported during July 2018–February 2020, 31 (94%) were caused by cVDPV2 (1).