1/ A strange thing about the flu is that at any given time only one strain of flu dominates. Why?

2/10 Influenza virus is highly mutable. New strains are churned out quite frequently. So much so that the flu virus is considered more as a quasi-species rather than a single virus. There are so many strains that it is a swarm.

3/10 The existing strains might have infected and granted some amount fo immunity to the hosts but new strains pop up all the time. And these new strains always dominate.

4/10 This is because when a new influenza virus emerges, it is highly competitive, even cannibalistic. It usually drives older types into extinction.

5/10 This happens because this new virus is very similar to the previous strains and the new infection stimulates the body’s immune system to generate all its defenses against all influenza viruses to which the body has ever been exposed.

6/10 This means that the older virus strains cant infect people anymore. They cannot gain a foothold in the host population and die out. Whereas the new strain, to which the immune response is not really effective propagates.

7/10 So, unlike practically every other known virus, only one type of influenza virus dominates at any given time.

8/10 And this cannibalism in a way helps the flu overall. Every new strain ensures that as more time passes the fewer people’s immune systems will recognize other antigens for older strains. So the chances of a new mutant strain that can sidestep the immune system are higher!

9/10 Thankfully the coronaviruses dont exhibit as much mutability nor this kind of strange memory erasure on our immune systems. So not really relevant for #COVID19 :)

10/10 This is based on some basic research and piqued by a discussion in The Great Influenza by John M Barry ( @PenguinBooks). Pls feel free to correct anything I might have got wrong...