CHICAGO—California on Thursday confirmed a fourth case of bird flu in a person who had contact with infected dairy cattle.

Nationwide, 18 people this year have tested positive for the virus, which has been confirmed in dairy herds in 14 states. All but one of the people were known to be exposed to sick cattle or poultry.

The four people who tested positive in California, the most populous U.S. state, were exposed to infected cows at four separate farms, the California Department of Public Health said.

The department added that it was notified of two additional possible human cases.

People who tested positive in California suffered mild symptoms, including eye redness or discharge, and were not hospitalized, the department said.