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.

Just how deadly is the H5N1 avian flu? The virus, which is currently sweeping through U.S. dairy herds, rarely jumps to human beings, at least for now. But when it does, the consequences can be grave: The World Health Organization reports that 52 percent of people known to be infected with H5N1 have died from the disease.