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.

San Mateo County health officials have announced the nation’s first known case of a severe form of mpox that has fueled an outbreak of the disease across Eastern and Central Africa.

San Mateo County health officials said on Saturday they have reported the first known case of clade I mpox in California and the United States.