  A two-dimensional lattice hard-core boson system with a small fraction of
bosonic or fermionic impurity particles is studied. The impurities have the
same hopping and interactions as the dominant bosons and their effects are
solely due to quantum statistics. Quantum Monte Carlo simulations are carried
out in which paths of the dominant boson species are sampled and a summation is
performed over all second-species paths compatible with the permutation cycles.
Both kinds of impurities reduce modestly and equally the Kosterliz-Thouless
superfluid transition temperature. However, the effective impurity interactions
are found to be qualitatively different at lower temperatures; fermions are
repulsive and further suppress superfluidity at low temperatures.
