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.

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.

Mystery H5 flu case had a close contact who was ill at same time; CDC hopes blood tests will show if there was a connection.

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.

WASHINGTON — Public health officials are still trying to determine how a Missouri resident contracted bird flu without having any contact with infected animals, but said Thursday there is no evidence of human-to-human transmission taking place in small enclaves or in a more widespread manner.

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.