  We compute the electric-current susceptibility \chi of hot quark-gluon matter
in an external magnetic field B. The difference between the susceptibilities
measured in the directions parallel and perpendicular to the magnetic field is
ultraviolet finite and given by \chi^\parallel-\chi^\perp = VTN_c\sum_f q_f^2
|q_f B|/(2\pi^2), where V denotes the volume, T the temperature, N_c the number
of colors, and q_f the charge of a quark of flavor f. This non-zero
susceptibility difference acts as a background to the Chiral Magnetic Effect,
i.e. the generation of electric current along the direction of magnetic field
in the presence of topological charge. We propose a description of the Chiral
Magnetic Effect that takes into account the fluctuations of electric current
quantified by the susceptibility. We find that our results are in agreement
with recent lattice QCD calculations. Our approach can be used to model the
azimuthal dependence of charge correlations observed in heavy ion collisions.
