Multiroute pathogen transmission is different

Epidemics are complex systems. They emerge from the interaction of host, pathogen, and environment. Consequently, since at least the time of Bernoulli, scientists and policy-makers have relied on data-driven models to under-stand how, when, and why small outbreaks grow into epidemics. Such models are necessary because the kinds of nonlinear, context-dependent, and unstable dynamics associated with epidemics—indeed all complex systems—are challenging to intuit. When pathogens transmit via multiple routes, e.g., sexual transmission and needle sharing, gaining an intuition for their dynamics without the use of models goes from challenging to nearly impossible.