The alpha, beta, gamma and delta variants are just some of the many thousands of different versions, or variants, of Covid-19 circulating around the world.
The longer the virus is spreading from person to person, the more likely it is that a new variant will appear and jeopardise vaccination programmes.
Some scientists are concerned that the newest Covid variant detected in South Africa, known as B.1.1.529, is “the worst one we’ve seen so far”.
But where are these variants coming from? Why do viruses mutate? And when does a variant become a “variant of concern”?
Health correspondent Laura Foster and virologist Dr Cindy Duke explain why and what we can do to stop it happening.
Video by Laura Foster