  Under the hypothesis that both influenza and respiratory syncytial virus
(RSV) are the two leading causes of acute respiratory infections (ARI), in this
paper we have used a standard two-pathogen epidemic model as a regressor to
explain, on a yearly basis, high season ARI data in terms of the contact rates
and initial conditions of the mathematical model. The rationale is that ARI
high season is a transient regime of a noisy system, e.g., the system is driven
away from equilibrium every year by fluctuations in variables such as humidity,
temperature, viral mutations and human behavior. Using the value of the
replacement number as a phenotypic trait associated to fitness, we provide
evidence that influenza and RSV coexists throughout the ARI high season through
superinfection.
