A Planet of Viruses

by Carl Zimmer

A quarter of all birds with the flu have two or more virus strains inside them at once. The viruses swap genes and gain new adaptations, such as the ability to move from living in wild birds to chickens, or even to mammals such as horses or pigs. And sometimes, on very rare occasions, reassortment can combine genes from avian and human viruses, creating a recipe for disaster. The new strain that results from this combination can easily spread from person to person. And because it has never circulated among humans before, no one has any defenses that could slow the new strain’s spread.