Where do you want to start? Viruses mutate very, very fast. Probably a new mutation appears about every day. There are already two strains in existence, but within those strains there are individual tiny mutations.
Not only corona virus but every virus is mutating every moment also. Many researchers are thinking that this COVID 19 came from the mutation of SARS COV. By mutation it can form another strain. Through mutation it can be more dangerous. By mutation it also can loss is activity.