  A susceptible-infected-susceptible (SIS) model of multiple contagions on
multilayer networks is developed to incorporate different spreading channels
and disease mutations. The basic reproduction number for this model is
estimated analytically. In a special case when considering only compartmental
models, we analytically analyze an example of a model with a mutation driven
strain persistence characterized by the absence of an epidemic threshold. This
model is not related to the network topology and can be observed in both
compartmental models and models on networks. The novel multiple-contagion SIS
model on a multilayer network could help in the understanding of other
spreading phenomena including communicable diseases, cultural characteristics,
addictions, or information spread through e-mail messages, web blogs, and
computer networks.
