Well, even if it mutates in a way that is beneficial (to us), you still have the original strain which will continue to spread unabated without a vaccine. It wa

Well, even if it mutates in a way that is beneficial (to us), you still have the original strain which will continue to spread unabated without a vaccine. It was just a handful of people who were infective initially, and now the diseases has spread everywhere. It can still happen again. Also, for diseases that are considered "eradicated" because we now have a vaccine, if you are one of the insane anti-vax people and stop giving vaccines to your kids you can still have outbreaks because of natural reservoirs for the disease. (For e.g. measles outbreak in the state of Washington). Infectious diseases rarely just go away completely.