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.

Federal officials are still trying to determine how a Missouri resident with no known exposure to bird flu, contracted the virus.

As of Friday, all that was known about the Missouri case was that the individual was infected with an H5 virus. The CDC was still working to try to figure out the neuraminidase of the virus, the N of its name. Sometimes that isn’t possible, for instance if there isn’t a lot of virus in the sample from the patient.

The concentration of viral RNA obtained from the Missouri patient who contracted H5 bird flu without any known exposure was so low that a full genetic sequence may not be available, federal health officials said.

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.