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.

In a disclosure that can't eliminate the possibility that bird flu may have spread from one human to another for the first time, U.S. health officials have reported that a person who lived with a Missouri resident infected with H5N1 became sick the same day.

Health officials say they don’t know how a Missouri person caught bird flu, but they believe it may be a rare instance of a “one-off,” stand-alone illness.

Federal officials are still trying to determine how a Missouri resident with no known exposure to bird flu, contracted the virus.

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.