CDC lab experts have found at least two variants of monkeypox virus circulating in this outbreak. Both variants share common ancestors with strains present in Nigeria since 2017. It is likely that there were at least two separate events where the monkeypox virus spread from animals to people in Nigeria, and then began to spread person-to-person through close contact. CDC lab experts are continuing to use genetic sequencing to better understand how cases might be linked to other cases in the US or in other countries.