The pattern has become all too familiar: A new version of the SARS-CoV-2 virus emerges and begins to dominate new Covid-19 cases, until it’s replaced by an even more contagious version of the virus.

This year, subvariants of the omicron variant of the virus have ruled cases in the US. The BA.1 subvariant started the omicron wave. Then in April, BA.2 formed the majority of cases. By May, BA.2.12.1 took over. Now BA.5 is in the lead, triggering a rise in hospitalizations across the country. It may be the most contagious version of the virus to date.

Why does this keep happening?

It’s evolution. The more a virus spreads, the more opportunities it has to mutate, and eventually some of those mutations will confer a transmission advantage to the virus.

Omicron showed that it was adept at causing reinfections among people who were previously exposed to Covid-19. BA.5 appears to have an especially potent mix of mutations that evade protection from the immune system.