  Opinion formation and disease spreading are among the most studied dynamical
processes on complex networks. In real societies, it is expected that these two
processes depend on and affect each other. However, little is known about the
effects of opinion dynamics over disease dynamics and vice versa, since most
studies treat them separately. In this work we study the dynamics of the voter
model for opinion formation intertwined with that of the contact process for
disease spreading, in a population of agents that interact via two types of
connections, social and contact. These two interacting dynamics take place on
two layers of networks, coupled through a fraction $q$ of links present in both
networks. The probability that an agent updates its state depends on both, the
opinion and disease states of the interacting partner. We find that the opinion
dynamics has striking consequences on the properties of disease spreading. The
most important is that the smooth (continuous) transition from a healthy to an
endemic phase observed in the contact process, becomes abrupt (discontinuous)
in the two-layer system. Therefore, disregarding the effects of social dynamics
on epidemics propagation may lead to a misestimation of the real magnitude of
the spreading. Also, an endemic-healthy discontinuous transition is found when
the coupling $q$ overcomes a threshold value. Furthermore, we show that the
disease dynamics delays the opinion consensus, leading to a consensus time that
varies non-monotonically with $q$ in a large range of the model's parameters. A
mean-field approach reveals that the coupled dynamics of opinions and disease
can be approximately described by the dynamics of the voter model decoupled
from that of the contact process, with effective probabilities of opinion and
disease transmission.
