EXAMPLE 15 Childhood Disease Epidemics

The incidence of childhood diseases such as chickenpox and measles varies greatly from year to year. Why?

There are four plausible explanations for the fluctuations:

  1. There is an underlying regular cycle that is perturbed and occasionally overwhelmed by random events (“noise”).
  2. There is no pattern, because the fluctuations are due solely to chance.
  3. There is no discernible pattern, because such fluctuations are inherent in the epidemiology of the disease, a chaotic system.
  4. Parents refuse to vaccinate their children against the disease, or the vaccine becomes less effective, thereby increasing the numbers of children susceptible.

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  • The first explanation, a perturbed cycle, fits chickenpox, with a cycle of one year.
  • For measles, either the second or the third explanation may be correct, depending on the size of the community. For small communities, chance is an adequate explanation. For large communities, historical data from before the era of mass immunization suggest that measles cases were chaotic. That doesn’t mean that they occurred at random, but rather that they were unpredictable. Research also shows that there is a critical community size above which a disease will not die out solely by chance. For measles, this size is about 250,000.
  • The two factors in the fourth explanation seems to account for recent flare-ups in whooping cough.