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 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.

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.