SARS-CoV-2, the virus that causes COVID-19.

https://www.yalemedicine.org/news/covid-19-variants-of-concern-omicron#:~:text=One%20thing%20we%20know%20for,Beta%2C%20Delta%2C%20and%20Omicron.?

One thing we know for sure about SARS-CoV-2, the virus that causes COVID-19, is that it is changing constantly. Since the beginning of the pandemic, we've seen a number of prominent variants, including Alpha, Beta, Delta, and Omicron.