  In this paper we present models for SSPs of intermediate and old ages where
dust enshrouded AGB stars are introduced. To this aim, particular care is
devoted to follow the evolution of the AGB stars throughout the quiet and
thermally pulsing regimes, to evaluate the effect of self contamination in the
outermost layers by the third dredge-up mechanism, to follow the transition
from oxygen-rich to carbon-rich objects, and finally to estimate the efficiency
of mass-loss by stellar winds. In addition to this, accurate physical models of
the dusty shells are presented in which the re-processing of radiation from the
central stars is calculated. The resulting spectral energy distribution (SED)
is examined to show how important features evolve with time. The SEDs are then
convolved with the IRAS filters to obtain the flux in various pass-bands for
individual AGB stars of different, mass, chemical composition, and age. The
comparison is made by means of SSPs along which AGB stars of the same age but
different initial masses are located. The theoretical results are compared to
the observational data for selected groups of stars. The same is made for the
J,H,K,L pass-bands of the Johnson system. Finally, from the integrated SEDs of
the SSPs, we derive the integrated Johnson J,H,K,L magnitudes and colors to be
compared to infrared data for star clusters of the Magellanic Clouds.
