Most cases of Polio are actually caused by vaccines - it's not a case of confusing cause and effect: https://en.wikipedia.org/wiki/Polio#Epidemiology

This is only the case because the vaccine has been so effective that the disease is close to being infected, and a vaccine-caused case is much less severe than a wild case, but they're still looking into a different vaccine type that will lower the risk of vaccine-caused infections.