  In this paper we develop an SIR model for coinfection. We discuss how the
underlying dynamics depends on the carrying capacity $K$: from a simple
dynamics to a more complicated. This can help in understanding of appearance of
more complicated dynamics, for example, chaos etc. The density dependent
population growth is also considered. It is presented that pathogens can invade
in population and their invasion depends on the carrying capacity $K$ which
shows that the progression of disease in population depends on carrying
capacity. Our approach is based on a bifurcation analysis which allows to
generalize considerably the previous Lotka-Volterra type models.
