Virus infections are normally transmitted by sap sucking insects such as greenfly and whitefly that move from plant to plant. Not a common problem, although potato and cucumber are susceptible to mosaic virus.
Virus is normally seen as spotting of the leaf, growth distortion or yellowing or mosaic patterning of leaves.
Treatment and control
General tips: First kill off any insect pests on affected and nearby plants with a pyrethrin based insecticide such as BugClear Gun! for Fruit & Veg to reduce spread of virus. Then dig up affected plant and throw it away.