With the approach of fall and cooler weather across the United States, officials say the risk posed by the H5N1 bird flu virus could rise — and they’re taking steps to prevent the creation of a hybrid flu virus that could more easily infect humans.

The world is watching how the U.S. is responding to the H5N1 bird flu outbreak in dairy cattle, a top WHO official said.

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.

The Missouri case raised questions about the possibility of human-to-human bird flu transmission, but officials said there is no evidence of other people being infected.

The case is concerning because it's the first human case this year not tied to a known exposure to animals. Before now, all positive cases came after a person came into contact with infected dairy cows or poultry.