The problem is that there's infinite ways that someone having a disease like polio could change their actions and therefore the future, and not all outcomes are actually worse. Perhaps having polio makes some people into jerks, and for others it somehow makes them better. Perhaps having polio effectively neutralizes some people who would have caused great harm if they had stayed healthy. Imagine if Hitler had contracted polio and was stuck in an iron lung for most of his life: history would be very different. Perhaps one of the people who did get stuck in an iron lung would have been another Hitler. The possibilities are endless.