  We study the two-pion propagator in the nuclear medium. This quantity appears
in the $\pi-\pi$ T-matrix and we show that it also enters the QCD scalar
susceptibility. The medium effects on this propagator are due to the influence
of the individual nucleon response to a scalar field through their pion clouds.
This response is appreciably increased by the nuclear environment. It produces
an important convergence effect between the scalar and pseudoscalar
susceptibilities, reflecting the reshaping of the scalar strengh observed in
$2\pi$ production experiments. While a large modification of the $\sigma$
propagator follows, due to its coupling to two pion states, we show that the NN
potential remains instead unaffected.
