  Many real networks are not isolated from each other but form networks of
networks, often interrelated in non trivial ways. Here, we analyze an epidemic
spreading process taking place on top of two interconnected complex networks.
We develop a heterogeneous mean field approach that allows us to calculate the
conditions for the emergence of an endemic state. Interestingly, a global
endemic state may arise in the coupled system even though the epidemics is not
able to propagate on each network separately, and even when the number of
coupling connections is small. Our analytic results are successfully confronted
against large-scale numerical simulations.
