The virus that causes COVID-19, the SARS-CoV-2 coronavirus, has a very simple genetic code made up of RNA. By comparison, you and I have much longer sequences of a different chemical called DNA in our genes. The virus is so simple that itâs hardly alive. On its own it canât do very much except survive. But, once it gets inside your cells, itâs able to hijack the infected cell and force it to make more copies of itself. It isnât capable of properly checking the copies being made and so there are always errors in the newly-made virusâ RNA. Most of these errors, or mutations, make no difference to how well the virus survives and spreads; some create a weaker virus; and there are even many that kill it. But, if enough copies are made, it makes sense that mutations can happen that give the virus an advantage.