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 shif

The CDC's analysis revealed changes in the hemagglutinin (HA) gene, a critical component for the virus's ability to attach to host cells. The mutations "were not found in virus sequences from poultry samples collected on the patient's property, suggesting the changes emerged in the patient after infection," the CDC stated in its December 26 update.