  If the background universe is observed through a significant amount of
diffusely distributed foreground dust, then studies at optical wavelengths may
be severely biased. Previous studies investigating effects of foreground dust
on background sources assumed the dust to be ``compactly'' distributed, ie. on
scales comparable to the visible extent of normal galaxies. We show however
that diffuse dust is more effective at obscuring background sources. Galaxy
clusters are a likely location for ``large-scale'' diffusely distributed dust,
and its effect on the counts of background sources is explored. We also explore
the implications of a hypothesised diffuse intergalactic dust component
uniformly distributed to high redshift with comoving mass density equal to that
associated with local galaxies. In this case, we predict a deficit in
background sources about three times greater than that found in previous
studies.
