What’s the significance of the 20 years? Is that how long it takes for the little buggers to mutate into a new strain? But surely that doen’t add up, because how could something like this be synchronised? Random mutations would presumably occur at random intervals all over the world; thus a new strain might develop in Hong Kong one year, another in Mexico a few years later, and so on. Even if they all took 20 years to complete the process, the differing mutations would be graduating in different years.
Flu viruses are mutating all the time. Twenty years is, on average, how long it takes random mutations to hit on a new variation that can ‘successfully’ (from the virus’s point of view) infect humans who have been immunized to previous strains. When such a ‘successful’ muttion occurs, it multiplies rapidly, as might be expected.