In countries that struggle with polio vaccination, we often use a "live attenuated" vaccine - that is, you are getting an infection with live virus, but one tha

In countries that struggle with polio vaccination, we often use a "live attenuated" vaccine - that is, you are getting an infection with live virus, but one that has been engineered to cause little to no disease. The advantages of this are that it's very easy to administer (this is the classic "vaccine dropper onto a kid's tongue" image), and because you shed the attenuated virus, you can help boost the immunity of those who aren't directly vaccinated. The downside is for the polio vaccine that it occasionally reverts back to a type that causes active disease. This is rare, but definitely happens, and this is one of those cases.