An extensive look at wastewater samples taken across the United States from May to July found traces of the H5N1 bird flu popping up -- but only in areas populated by farm animals.

A person in Missouri is the United States’ first case of H5 avian influenza without a known exposure to a sick animal, the US Centers for Disease Control and Prevention said on Friday.

A close contact of someone with bird flu became ill on the same day, the C.D.C. reported. But the second person was not tested, and the cause of the illness is unknown.

Genetic analysis of a bird flu virus detected in a person in Missouri who didn’t previously have contact with animals offers more details on the case, but experts say there isn’t substantial evidence

Last week, a person in Missouri was infected with bird flu despite not having any exposure to animals. The state’s health department said the individual recovered from their illness but that a household contact of the patient had become ill with similar symptoms on the same day.

San Diego County is now the fifth location in California to detect the bird flu, county officials announced Monday.