Syphilis Cycles

Syphilis has re-emerged as a global public health issue. In lesser developed countries, millions of people are contracting the disease, which can be fatal without access to proper treatment. In developed countries, prevalence is much lower but has cycled around endemic levels for decades. The authors of a recent high-profile article in the journal Nature argue that these regular fluctuations in syphilis prevalence are driven primarily by endogenous disease dynamics rather than social or behavioral factors, as often theorized. We explore this hypothesis by extending the classic SIRS epidemiological model to incorporate forward-looking, rational individuals. This economic SIRS model (or E-SIRS) model is consistent with microeconomic fundamentals as it is derived from the behavioral equations of rational individuals. In contrast to the Nature article, the E-SIRS model predicts that human preferences over health and sexual activity are central to the nature of syphilis cycles. We find that low-activity individuals will behave in a manner that significantly dampen the cycles, while high-activity individuals will tend to exacerbate the cycles, a phenomenon we refer to as rational dynamic resonance. The economic SIRS model also provides additional insights into two failed attempts by the U.S. government to eradicate syphilis from the U.S. population.