  We prove H\"older regularity for a general class of parabolic
integro-differential equations, which (strictly) includes many previous
results. We present a proof which avoids the use of a convex envelop as well as
give a new covering argument which is better suited to the fractional order
setting. Our main result involves a class of kernels which may contain a
singular measure, may vanish at some points, and are not required to be
symmetric. This new generality of integro-differential operators opens the door
to further applications of the theory, including some regularization estimates
for the Boltzmann equation.
