Viruses simply diversify through mutation. They can be more or less infectious, more or less deadly, etc in any combination and selection pressure over the long

Viruses simply diversify through mutation. They can be more or less infectious, more or less deadly, etc in any combination and selection pressure over the long term tends to lead to less deadly more infectious variants dominating. In the mean time more dangerous variants can do a lot of damage. What the Spanish Flu, most deaths occurred in the second and third waves from more virulent and deadly variants.