Influenza viruses like bird flu can mix and match their genomes, and this has played a role in at least three of the last four flu pandemics. Influenza viruses are shifty entities. They accumulate small genetic changes on a regular basis, necessitating yearly updates to the flu vaccines because the prior year’s strain may not look much like the following year’s. But they can also make sudden leaps by incurring big genetic changes that may allow them to jump from one animal species to another or to humans.
A seemingly ingenious and sneaky way for viruses to make these leaps is by swapping genetic material with other flu strains. Called reassortment, this exchange happens when a person or animal is infected with two types of flu virus at the same time. While replicating inside the host cell, the viruses can grab bits of each other’s genetic code and incorporate them into their own gene sequences.