This work is aimed at ascertaining the impact of an opportunistic disease outbreak in a population already affected by a primary disease.

We are interested in possible synergistic affects and we found that the fate
of the primary disease epidemic process may change drastically when
the opportunistic disease enters the scene. Our main assumption is that the primary disease runs for a long time and weakens the host organism allowing the invasion of the opportunistic disease.

In spite of the impact of opportunistic diseases in public health, there is a lack of mathematical models devoted to understand this phenomenon. A major difficulty in the study of such kind of models is that the resulting system consist of a large number of coupled equations.

Approximate aggregation techniques take advantage on the time scales separation
to get a reduced system of lower dimension attaining asymptotic information on the complete model.