A patient in Missouri who was hospitalized after an infection with bird flu had the H5N1 strain of the virus, the US Centers for Disease Control and Prevention confirmed Friday.

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.

The patient reported no contact with animals, leaving experts wondering how they contracted the flu — and whether the virus is now traveling between humans.

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

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.

The Czech Republic has reported an outbreak of the highly pathogenic H5N1 strain of avian influenza, or bird flu, on a poultry farm, the World Organisation for Animal Health (WOAH) said on Monday.