  Interacting fluids, endowed with bulk viscous stresses, are discussed in a
unified perspective with the aim of generalizing the treatment of cosmological
perturbation theory to the case where both fluctuating decay rates and
fluctuating bulk viscosity coefficients are simultaneously present in the
relativistic plasma. A gauge-invariant treatment of the qualitatively new
phenomena arising in this context is provided. In a complementary approach,
faithful gauge-fixed descriptions of the gravitational and hydrodynamical
fluctuations are developed and exploited. To deepen the interplay between bulk
viscous stresses and fluctuating decay rates, illustrative examples are
proposed and discussed both analytically and numerically. Particular attention
is paid to the coupled evolution of curvature and entropy fluctuations when, in
the relativistic plasma, at least one of the interacting fluids possesses a
fluctuating bulk viscosity coefficient. It is argued that this class of models
may be usefully employed as an effective description of the decay of the
inflaton as well as of other phenomena involving imperfect relativistic fluids.
