In other developments, CDC and Ohio partners launched a serosurvey among bovine veterinary workers who attended a recent professional conference.

Another health care worker who was exposed to a Missouri patient who tested positive for bird flu developed respiratory symptoms but wasn't tested for the flu, U.S. health officials reported 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.

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.

In other developments, CDC and Ohio partners launched a serosurvey among bovine veterinary workers who attended a recent professional conference.

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 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.

A second 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.