They are adept hitchhikers, traveling in the gut of certain insects, nematodes, even fungi, then silently slipping into a plant cell as any of these organisms begin feeding on a plant. One way, then, to prevent virus infections in plants is to control insects or other organisms that spread viruses. Another control is to isolate cultivated plants from related wild plants and weeds, both of which might harbor viruses. Aphids, for example, can carry cucumber mosaic virus from ragweed or milkweed to cucumbers, or raspberry mosaic virus from wild raspberries to cultivated raspberries. Rip out any cultivated plant known to be infected with a virus.
Viruses even solicit rides from humans.
Tobacco mosaic virus infects a wide variety of plants, and can be carried on smokers' fingers. Plant choice can help avoid virus problems. Some varieties are virus-resistant. Don't blame all your plant problems on viruses. Definitive diagnosis is often difficult without a laboratory test.