  We use the method of sliding paraboloids to establish a Harnack inequality
for linear, degenerate and singular elliptic equation with unbounded lower
order terms. The equations we consider include uniformly elliptic equations and
linearized Monge-Amp\`ere equations. Our argument allows us to prove the
doubling estimate for functions which, at points of large gradient, are
solutions of (degenerate and singular) elliptic equations with unbounded drift.
