  Particle-hole modes with quantum numbers of pions and negative kaons can
propagate in nuclear matter. We discuss possible manifestations of these modes
in experiments on heavy-ion collisions and on neutrino-nucleus scattering.
Calculations of reaction rates in medium can be harmed by double counting,
which arises because the very same quantum numbers can be carried in medium as
by single particle excitations as by multi-particle ones. We argue that the
optical theorem written in terms of the non-equilibrium Green's functions
provides a convenient formalism (closed diagram technique) void of double
counting.
