Evolving Epidemiology

ARTICLE EXTRAS

1 Epidemic patterns are largely determined by the time course of an infection, whether it's measured in days, weeks, or years. Phylogenetic patterns result from an interaction between natural selection mediated by the immune system (and/or drug treatments) and random epidemiological processes. Here's how some common pandemics play out.

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...