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.

A second health worker in contact with the Missouri patient who tested positive for bird flu also developed symptoms but wasn’t tested for influenza, the CDC said Friday.

A third close contact of a patient in Missouri who was hospitalized with H5N1 bird flu has reported that they also experienced symptoms, the US Centers for Disease Control and Prevention said Friday.

Health officials identified three contacts, including two healthcare workers, who were closely monitored after a bird flu patient in Missouri was recently hospitalized.

A second health worker who cared for a person hospitalized in Missouri with H5N1 bird flu developed mild respiratory symptoms but was not tested for influenza, the CDC reported.

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.

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

Missouri healthcare worker who had contact with a hospitalized patient infected with bird flu developed mild respiratory symptoms but was not tested for the virus, U.S. officials said on Friday.