  We extract the bulk viscosity of hot quark-gluon matter in the presence of
light quarks from the recent lattice data on the QCD equation of state. For
that purpose we extend the sum rule analysis by including the contribution of
light quarks. We also discuss the universal properties of bulk viscosity in the
vicinity of a second order phase transition, as it might occur in the chiral
limit of QCD at fixed strange quark mass and most likely does occur in
two-flavor QCD. We point out that a chiral transition in the O(4) universality
class at zero baryon density as well as the transition at the chiral critical
point which belongs to the Z(2) universality class both lead to the critical
behavior of bulk viscosity. In particular, the latter universality class
implies the divergence of the bulk viscosity, which may be used as a signature
of the critical point. We discuss the physical picture behind the dramatic
increase of bulk viscosity seen in our analysis, and devise possible
experimental tests of related phenomena.
