We develop an efficient Monte-Carlo approach to describe the optical response
of cold three-level atoms in the presence of EIT and strong atomic
interactions. In particular, we consider a "Rydberg-EIT medium" where one
involved level is subject to large shifts due to strong van der Waals
interactions with surrounding Rydberg atoms. We find excellent agreement with
much more involved quantum calculations and demonstrate its applicability over
a wide range of densities and interaction strengths. The calculations show that
the nonlinear absorption due to Rydberg-Rydberg atom interactions exhibits
universal behavior
