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Flu viruses are composed of eight segments, including the haemagglutinin protein, or “H” part of the virus, and neuraminidase, or the “N” part.

When an animal is infected by two different flu viruses, the genetic material of the two can switch or recombine to form a new version — which is what probably happened here: While the haemagglutinin — the part of the virus that allows it to attach to a host — remained the same, a new neuraminidase — the part that helps release the virus from infected cells — was substituted in.

In the process it acquired a new neuraminidase, the N in H5N1 and the protein the virus uses to detach new virus particles from the cell that produced them.

The two proteins have opposing effects on the same sugar chains on the surface of human cells: Hemagglutinin attaches to these chains, helping the virus infect new cells, whereas neuraminidase cuts those chains, freeing newly formed virus from host cells.

“If your hemagglutinin is too sticky and your neuraminidase is too poor, you get stuck to the cell that you’ve just budded from,” Peacock says.