  Epidemic spread in single-host systems strongly depends on the population's
contact network. However, little is known regarding the spread of epidemics
across networks representing populations of multiple hosts. We explored
cross-species transmission in a multilayer network where layers represent
populations of two distinct hosts, and disease can spread across intralayer
(within-host) and interlayer (between-host) edges. We developed an analytic
framework for the SIR epidemic model to examine the effect of (i) source of
infection and (ii) between-host asymmetry in infection probabilities, on
disease risk. We measured risk as outbreak probability and outbreak size in a
focal host, represented by one network layer. Numeric simulations were used to
validate the analytic formulations. We found that outbreak probability is
determined by a complex interaction between source of infection and
between-host infection probabilities, whereas outbreak size is mainly affected
by the non-focal host to focal host infection probability alone. Hence,
inter-specific asymmetry in infection probabilities shapes disease dynamics in
multihost networks. These results expand current theory of monolayer networks,
where outbreak size and probability are considered equal, highlighting the
importance of considering multiple measures of disease risk. Our study advances
understanding of multihost systems and non-biological systems with asymmetric
flow rates.
