  In ecological systems heterogeneous interactions between pathogens take place
simultaneously. This occurs, for instance, when two pathogens cooperate, while
at the same time multiple strains of these pathogens co-circulate and compete.
Notable examples include the cooperation of HIV with antibiotic-resistant and
susceptible strains of tuberculosis, or some respiratory infections with
Streptococcus pneumoniae strains. Models focusing on competition or cooperation
separately fail to describe how these concurrent interactions shape the
epidemiology of such diseases. We studied this problem considering two
cooperating pathogens, where one pathogen is further structured in two strains.
The spreading follows a susceptible-infected-susceptible process and the
strains differ in transmissibility and extent of cooperation with the other
pathogen. We combined a mean-field stability analysis with stochastic
simulations on networks considering both well-mixed and structured populations.
We observed the emergence of a complex phase diagram, where the conditions for
the less transmissible, but more cooperative strain to dominate are
non-trivial, e.g. non-monotonic boundaries and bistability. Coupled with
community structure, the presence of the cooperative pathogen enables the
co-existence between strains by breaking the spatial symmetry and dynamically
creating different ecological niches. These results shed light on ecological
mechanisms that may impact the epidemiology of diseases of public health
concern.
