  In this paper we present a method to compute the exact density of states
induced by $N$ non magnetic impurities in a system of two dimensional Dirac
fermions in the unitarity limit. We review the case of the $\pi$-flux phase of
the Heisenberg model and also treat the disordered d-wave superconductor. In
both case we find additional states in the gap with $\delta \rho (\omega)
\simeq n_i/ |\omega(\ln^2 |\omega/\Delta_0| + (\pi/2)^2)|$.
