  We study linear time fractional diffusion equations in divergence form of
time order less than one. It is merely assumed that the coefficients are
measurable and bounded, and that they satisfy a uniform parabolicity condition.
As the main result we establish for nonnegative weak supersolutions of such
problems a weak Harnack inequality with optimal critical exponent. The proof
relies on new a priori estimates for time fractional problems and uses Moser's
iteration technique and an abstract lemma of Bombieri and Giusti, the latter
allowing to avoid the rather technically involved approach via BMO. As
applications of the weak Harnack inequality we establish the strong maximum
principle, continuity of weak solutions at t = 0, and a Liouville type theorem.
