  We examine the biases induced on cosmological parameters when the presence of
secondary anisotropies is not taken into account in Cosmic Microwave Background
analyses. We first develop an exact analytical expression for computing the
biases on parameters when any additive signal is neglected in the analysis. We
then apply it in the context of the forthcoming Planck experiment. For
illustration, we investigate the effect of the sole residual thermal
Sunyaev--Zel'dovich signal that remains after cluster extraction. We find in
particular that analyses neglecting the presence of this contribution introduce
on the cosmological parameters n_s and tau biases, at least 6.5 and 2.9 times
their one sigma confidence intervals. The Omega_b parameter is also biased to a
lesser extent.
