In each warm body it infects, the virus behind Covid-19 has the potential to change. It can become more deadly, more transmissible or more resistant to the vaccines on which we are all pinning so much hope. Mercifully, the biology of Sars-CoV-2 means that such changes happen slowly and almost always fail to catch on.
But mutations, like pandemics, are a numbers game. Every new person infected provides another opportunity for the virus to adopt a new form. So far, Sars-CoV-2 has infected at least 106 million people worldwide and taken on many thousands of mutations. Most of those changes are slow and inconsequential – evolutionary dead ends that nobody will ever realise existed. But, in some people, the virus hits the jackpot.
