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.

A routine flu screening in late summer found the H5N1 virus in a Missouri patient. Unlike the other reported cases of avian flu, this person did not report being in recent contact with animals.

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.

The latest developments are highly concerning, as they raise the possibility that the H5N1 virus has evolved the ability to transmit from human-to-human and thus the potential to cause the next pandemic.

Chances are low that human-to-human spread of bird flu occurred in a group of people who were exposed to a patient infected with the virus in Missouri, US health authorities said.

At least one household member and six healthcare workers who encountered the index case-patient are being investigated after showing symptoms of viral infection.

Missouri officials are under growing pressure from national public health experts to pinpoint the source of the state’s first human bird flu infection.

The CDC said an investigation into a human H5N1 bird flu infection in Missouri has found four additional health care workers who fell ill after caring for the patient.

In what could be the first cases of bird flu spreading between humans in the United States, a group of potential H5N1 infections in Missouri has now grown to eight. However, state health officials are still awaiting antibody tests to confirm whether the H5N1 infection is growing,

After a patient with bird flu was hospitalized last month, state and federal health officials first determined the patient may have infected one household member and two hospital workers. However, "Missouri has since identified four additional health care workers who later developed mild respiratory symptoms,