Human prophylaxis driven by risk may cause oscillations in sexually transmitted diseases
Infectious diseases often display oscillations in the number of infected cases through time. Sometimes the ups and downs are caused by seasonal, exogenous events, such as the increase of influenza cases in winter, or the increase of vector-borne diseases during rainy seasons. Other times, the infection displays non-seasonal periodic oscillations, like syphilis, which oscillates with a period of 8-11 years. Several mathematical models aim to capture these oscillations, either by including a period in the transmission rate, by allowing link rewiring in contact networks, or by considering models with temporary immunization. Here we present a stochastic, yet analytically solvable, epidemic spreading model where the individuals of the population decide whether to take preventive measures or not depending on the global extent of the disease, being this an assessment of their infection risk. We show that the combined feedback between the human decision on prophylaxis, and the perceived epidemic risk, are sufficient conditions for the emergence of self-sustained oscillations in diseases well-described by the Susceptible-Infected-Susceptible (SIS) compartmental model, as it is the case for many sexually transmitted diseases (STDs). Finally, we propose plausible mechanisms to damp out the oscillations. Our study prompts to the design of persistent prevention campaigns, substantiated on not only perceived but real risks, to improve human prophylactic behavior and contain the recently reported raise of STDs and the consequent re-emergence of HIV.