Flu’s success owes much to its genetic mutability

BOTH “mini-epidemics” of seasonal flu, which happen most years, and much larger pandemics, of which 1918’s was the worst example, are the result of an arms race between the influenza virus and the immune systems of the animals it infects. Here’s how it works.Type A flu viruses—those which cause pandemics, and also most seasonal flu, have two important surface proteins, haemagglutinin (H) and neuraminidase (N). Haemagglutinin helps the virus invade a target cell. Neuraminidase helps new virus particles break out of that cell. These two proteins are also antigens, meaning that they are parts of a virus that may be recognised and reacted to by the immune system.