  In principle every excitation acquires a finite lifetime in a hot system.
This nonzero spectral width is calculated self-consistently for massive
fermions coupled to massless scalar, vector and pseudoscalar bosons. It is
shown that the self-consistent summation of the corresponding Fock diagram for
fermions eliminates all infrared divergences although the bosons are not
screened at all. Our solutions for the fermion damping rate are analytical in
the coupling constant, but not analytical in the temperature parameter around
T=0.
