  Traditional biomedical approaches treat diseases in isolation, but the
importance of synergistic disease interactions is now recognized. As a first
step we present and analyze a simple coinfection model for two diseases
affecting simultaneously a population. The host population is affected by the
\emph{primary disease}, a long-term infection whose dynamics is described by a
SIS model with demography, which facilitates individuals acquiring a second
disease, \emph{secondary (or \emph{opportunistic}) disease}. The secondary
disease is instead a short-term infection affecting only the primary-infected
individuals. Its dynamics is also represented by a SIS model with no
demography. To distinguish between short and long-term infection the complete
model is written as a two time scales system. The primary disease acts at the
slow time scale while the secondary disease does at the fast one, allowing a
dimension reduction of the system and making its analysis tractable. We show
that an opportunistic disease outbreak might change drastically the outcome of
the primary epidemic process, although it does among the outcomes allowed by
the primary disease. We have found situations in which either acting on the
opportunistic disease transmission or recovery rates or controlling the
susceptible and infected population size allow to eradicate/promote disease
endemicity.
