I've been led to understand that this was usually the other way around, or that the mechanism that allowed their survival in the first place was "change something in the immune system, see if it works, repeat until it does". Through some magical process of biology or chemistry afterwards, the found solution is then "remembered" and ready to be deployed again if the disease returns.

Oh, yeah, I know about that. I understood that it didn't work on everything, though. (Well, it doesn't work on the common cold, for a start, although I'm not sure if that kind of constant low-level mutation is feasible for more ... powerful ... diseases.

EDIT: turns out it is.