  The nucleon mass shift is calculated using chiral counting arguments and a
virial expansion, without and with the $\Delta$. At all temperatures, the mass
shift and damping rate are dominated by the $\Delta$. Our results are compared
with the empirical analysis of Leutwyler and Smilga, as well as results from
heavy baryon chiral perturbation theory in the large $N_{c}$ (number of color)
limit. We show that unitarity implies that the concepts of thermal shifts are
process dependent.
