 Virus mutations happen when the virus is making copies of itself inside of our cells. When the virus infects us, it attaches to the outside of our cell and then works its way to the inside. Once inside, its job is to make multiple copies of itself to then be released to go infect more cells. It makes those copies by copying its genetic material, called RNA, so that that can become the new viruses. When it's doing that copying, it can make errors and when it makes those mistakes, the genetic material is changed. That results in a change in the new viruses, either in its structure or function and how it behaves. And this can lead to changes in how easily it's transmitted or spread, how easily and how much disease it causes, and how well it responds to vaccines and to different treatments.