It happens. I had chicken pox twice, the second time at the ripe age of 28 which was rather unexpected. It's not nice to be a statistical outlier!
I had German measles twice. I'm not aware that mutates like flu and covid.
It's a virus, all viruses mutate. All living things mutate. The genetically simpler the species the more effect a mutation will have.
It takes several, perhaps 10-30, generations for an advantageous mutation in a species to become a distinct strain. The problem, from our point of view, is that a generation for many microbes is on the close order of 40 minutes. The good thing is 99% of all microbe mutations are lethal to the mutating species (the downside of genetic simplicity, every gene is vital), 0.99999% are not immediately lethal but are still disadvantageous, 0.0000999% confer a slight advantage and 0.00000001% are really advantageous (percentages made up, but they're on that order). The other bad thing is that the numbers of microbes involved in an individual infection (bacterial or viral) number in the billions, so the tiny probabilities of advantageous mutations are balanced somewhat by the sheer numbers of microbes involved.
The inevitable outcome, especially in a widespread outbreak of an infectious disease, is that new strains are born all the time.