The case marked the first known instance of that particular mpox variant appearing in the United States. A different variant of the virus, called clade II, has been circulating in the U.S. since 2022 and has sickened at least 108 people in the county, the agency said.

The first case in the U.S. of a more severe strain of mpox was reported in San Mateo County, the California Department of Public Health said.

The county of San Mateo has now had the first reported case of clade I mpox in the United States, according to the California Department of Public Health (CPDH). What is mpox? Mpox is an infectious disease that can cause a painful rash,