The convergence of multiple flu strains in one animal can allow new virus strains to form—just as they did in past pandemics.

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.

As the fall and winter seasons approach the US, scientists are worried about a renewed spread of the deadly H5N1 bird flu virus, which had somehow slowed down in the summer. According to experts, a hybrid virus could more easily infect humans as the upcoming weather would see more movement of cows for grazing.

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.