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.

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.

Health officials on Thursday said they don’t know how a Missouri person caught bird flu but believe it may be a rare instance of a “one-off" standalone illness. Investigators trying to determine how the person caught the virus have not been able to confirm the exact strain of flu.

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.

An adult with underlying medical conditions was hospitalized on Aug. 22 and has since recovered, the Missouri health department said.

One person is now confirmed to have contracted a case of the avian flu virus, or H5N1, in Missouri, the U.S. Centers for Disease Control and Prevention confirmed.

A day after the Centers for Disease Control and Prevention said it had yet to identify “a clear source” of infection in a Missouri patient who tested positive for the bird flu virus, the agency quietly disclosed in its weekly influenza report that a close contact was sick around the same time as the Missouri patient but was not tested for influenza.

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.

A close contact of a Missouri resident identified as having a human case of the H5N1 avian flu also became sick around the same time, the Centers for Disease Control and Prevention has revealed.