Figure 2. Epidemic cycles with varied infectious periods and latent periods.

Short latent and infectious periods are required for epidemic cycles longer than one year (the periodicity of school terms). The latent and infectious periods for MP are both 2 to 3 weeks. Black denotes annual epidemic cycles and white denotes epidemic cycles longer than one year. To explore the possibility of oscillation the epidemic cycles were calculated with varied b from 1.1 to 17.0 (the estimated b for measles cases in the U.K. is 1.7²⁷). Mean duration of immunity is assumed to be 7 years.