Viruses always evolve to be more contagious if they can, while at the same time they evolve to being less virulent/pathogenic, especially respiratory viruses.

In their evolution, each virus strives to grab “market share” for its progeny. The best way to achieve this is to get host cells in the human body to generate as many copies of itself as possible, while at the same time not make that person so ill that they meet fewer people,

because if a sick person were too ill, then not be out and about, and end up not interacting with other people as much, the virus risks killing its host or itself becoming “killed” by its host (the human) before enabling its decendants the chance to relocate into another host....