Not an expert, but from hearing about it from various sources: As viruses evolve they generally have 2 paths of evolution – they either become more deadly, or

Not an expert, but from hearing about it from various sources: As viruses evolve they generally have 2 paths of evolution – they either become more deadly, or more contagious. The ones that become more deadly will kill the host quicker, and so won't have enough time to spread to others. The ones that become more contagious are by definition weaker, they will spread to more people, but the effects will be milder. There are some theories to suggest that pandemics (like the Spanish flu) ended because the virus eventually mutated to a milder, more contagious form.