ARTICLE EXTRAS
Measles is a highly contagious infection lasting an average of 12 days. Infection (or vaccination) confers lifelong immunity to all strains of the virus, although the reason for this is unclear. The observed biannual epidemic cycles occur as the number of susceptible hosts are repeatedly exhausted and replenished with a new generation of schoolchildren. Nevertheless, the battle between the virus and host never escalates because natural selection operates equally on all strains. No single strain of measles predominates, and around the globe a number of strains are able to coexist.
Influenza A also has a short infection period but differs from measles in that there is no cross-immunity between different strains of the virus. Each year random mutation produces a large genetic diversity that doesn't necessarily change the face of the virus to the immune system. Suddenly, a mutation results in a large shift in the shape ...
