Epidemic Cycles Driven by Host Behavior

Many infectious diseases cycle in incidence. Host immunity and demographics (the recruitment of susceptibles via birthrate) have been demonstrated to be a key determinant of the periodicity of measles [1], pertussis [2], and dengue epidemics [3, 4]. For diseases which induce strong sterilizing immunity, models incorporating demographics and host immunity offer parsimonious explanations to the observed cycles in incidence. However, not all epidemic cycles are from pathogens inducing sterilizing immunity or are driven by demographics. Seasonal cycles of measles and other childhood microparasitic infections are driven by birth rates, age-specific contact rates and school-age mixing [5], and many sexually transmitted infections (STIs) are driven by sexual behavior. Here we show that cycles in syphilis (Treponema pallidum) incidence can be solely due to adaptive behavior in structured populations. We present a mathematical model which shows how such dynamics can exhibit both periodicity or lack thereof and are more sensitive to changes in behavior than biological parameters. This carries important consequences for the control and potential eradication of infectious diseases [6].