  We present a theory describing spiral magnets with Dzyaloshinskii-Moriya
interaction (DMI) subject to bond disorder at small concentration $c$ of
defects. It is assumed that both DMI and exchange coupling are changed on
imperfect bonds. Qualitatively the same physical picture is obtained in two
models which are considered in detail: B20 cubic helimagnets and layered
magnets in which DMI leads to a long-period spiral ordering perpendicular to
layers. We find that the distortion of the spiral magnetic ordering around a
single imperfect bond is long-range: values of additional turns of spins decay
with the distance $r$ to the defect as $1/r^2$ being governed by the Poisson's
equation for electric dipole. At finite concentration of randomly distributed
imperfect bonds, we calculate correction to the spiral vector. We show that
this correction can change the sign of spin chirality even at $c\ll1$ if
defects are strong enough. It is demonstrated that impurities lead to a diffuse
elastic neutron scattering which has power-law singularities at magnetic Bragg
peaks positions. Then, each Bragg peak acquires power-law decaying tails.
Corrections are calculated to the magnon energy and to its damping caused by
scattering on impurities.
