“For example, if a human gets infected with a bird flu and also carries a human influenza A virus, these two viruses can exchange genetic material. This is known as genetic shift,” Michael previously told Fortune. “That can form very new viruses [and] cause epidemics.”

Bird flu cases are on the rise in the U.S., and the CDC is on high alert. With 66 confirmed human cases so far, officials are watching closely for signs that the virus could mutate and spread more easily between people,

The Centers for Disease Control and Prevention (CDC) has identified mutations in the H5N1 bird flu virus that may increase its ability to infect humans. These mutations, found in the hemagglutinin (HA) gene,