  This study is concerned with the dynamical behaviors of epidemic spreading
over a two-layered interconnected network. Three models in different levels are
proposed to describe cooperative spreading processes over the interconnected
network, wherein the disease in one network can spread to the other.
Theoretical analysis is provided for each model to reveal that the global
epidemic threshold in the interconnected network is not larger than the
epidemic thresholds for the two isolated layered networks. In particular, in an
interconnected homogenous network, detailed theoretical analysis is presented,
which allows quick and accurate calculations of the global epidemic threshold.
Moreover, in an interconnected heterogeneous network with inter-layer
correlation between node degrees, it is found that the inter-layer correlation
coefficient has little impact on the epidemic threshold, but has significant
impact on the total prevalence. Simulations further verify the analytical
results, showing that cooperative epidemic processes promote the spreading of
diseases.
