  We study the effective Lagrangian, at leading order in derivatives, that
describes the propagation of density and metric fluctuations in a fluid
composed by an arbitrary number of interacting components. Our results can be
applied to any situation in cosmology where different species have
non-gravitational interactions. In time-dependent backgrounds, such as FLRW,
the quadratic action can only be diagonalized at fixed time slices and flavour
mixing is unavoidable as time flows. In spite of this, the fluid can be
interpreted at the level of the linear equations of motion as an ensemble of
individual interacting species. We show that interactions lead to anisotropic
stresses that depend on the mixing terms in the quadratic action for
perturbations. In addition to the standard entrainment among different
components, we find a new operator in the effective action that behaves as a
cosmological constant when it dominates the dynamics.
