Before H5N1 can be transmitted human-to-human a "crossover event" has to occur:
How a Virus Crosses Over from Animals to Humans A crossover event, where a virus adapts from infecting animals to infecting humans, is a complex process that involves several critical changes. It’s not as simple as a single mutation; it requires a series of adaptations for the virus to overcome species barriers and spread efficiently among humans. Here’s what typically needs to happen: 1. Receptor Binding Adaptation Viruses like avian influenza are designed to bind to specific receptors in their host. In birds, the virus binds to α2-3-linked sialic acid receptors. Humans, however, have α2-6-linked sialic acid receptors in their respiratory tract. For the virus to infect humans, it must mutate its surface proteins—particularly hemagglutinin (HA)—to attach to human receptors. This is a key step in crossing the species barrier. 2. Enhanced Replication in Human Cells Once inside human cells, the virus must replicate efficiently. This often r...