  Sterile neutrinos are one of the most promising Warm Dark Matter candidates.
By considering their radiative- and pion-decay channels, we derive the allowed
contribution of sterile neutrinos to the X-ray, optical and near-infrared
cosmic backgrounds. The X-ray background puts a strong constraint on the mass
of radiatively decaying neutrinos (m <= 14 keV), whereas the allowed mass range
for pion-decay neutrinos (for a particle lifetime > 4 X 10^17 s) is 150 <=
m/MeV <= 500. Taking into account these constraints, we find that sterile
neutrinos do not significantly contribute to the optical and near-infrared
background. We further consider the impact of sterile neutrinos on
reionization. We find that the Thomson optical depth due to sterile neutrinos
is tau_e = (0.4-3) X 10^-2 in the case of radiative decays, and it is ~10^-3
for the pion-decay channel. We conclude that these particles must have played
only a minor role in cosmic reionization history.
