I don’t think people understand what mutation is.
1) The virus is not alive. Mutation is not a process of life.
2) Mutation is a corruption of the RNA segment of the virus. Some viruses have very stable, easy to copy segments. Others are unstable and replication introduces “errors”.
3) When a virus invades a cell, the cell’s function changes to whatever it was before to one of replicating the virus. The replication step is where the stable or non stable RNA issue arises. If unstable, “errors” occur and the produced virus will not be the same as the invading virus.
4) The unstable RNA replication creates a virus that is a mutation. It then bursts out of the cell and, in the lungs, floats around in the air inside the lung until it bumps into another new cell to invade. Note it does not “go looking for a new cell” and it does not “grab a new cell”. It’s not alive. It just floats around and bumps into things.
5) Smallpox was a very stable RNA sequence. Replication was always the same. Few or no mutations. That’s why it was defeated by vaccine.
6) Not so corona.
Mutations are always detrimental. Never beneficial to the virus