If viruses have a purpose, it’s to make copies of themselves. They don’t have the tools to do that on their own, so they have to hijack cells from a host (i.e., us) in order to reproduce. The copying process can be sloppy, especially with viruses that use RNA as their genetic material, like SARS-CoV-2, so mutations abound.
Most of these changes are detrimental to the virus or have no effect, but some can make the virus cause more severe disease, infect more people, or better hide from the immune system. When lots of people have been vaccinated or previously infected, mutations that conceal the virus have a huge advantage.
“The high level of immunity in the population is likely exerting selection pressure on the virus and the virus is evolving to try to get around that immunity,” said Daniel Barouch, director of the Center for Virology and Vaccine Research at the Beth Israel Deaconess Medical Center.