The Centers for Disease Control and Prevention should have results later this month that provide more insight into how a Missouri resident, who hadn’t had any contact with infected animals or food, contracted a case of highly pathogenic avian influenza.

Hospital workers reported respiratory symptoms after encountering a Missouri patient with H5N1 who had not been exposed to farm animals. Officials say bird flu transmission is unlikely.

U.S. health officials have run into obstacles in their efforts to determine whether a Missouri person infected with H5N1 bird flu passed the virus on to others.