Health authorities in the US are studying seven people who developed influenza symptoms after being exposed to a Missouri bird flu patient, raising the possibility of the first human-to-human transmission of the infection.

Seven people in contact with a patient hospitalized with bird flu also developed symptoms, the C.D.C. reported. Some are undergoing further tests.

Health officials are investigating whether a person in Missouri who was hospitalized with bird flu in early September might have infected a handful of others. Testing is ongoing among five health care workers and a household member exposed to the infected person who all came down with mild respiratory symptoms.