Infection of plants by viruses can lead to a systemic or a localized infection. Systemic infections can produce protection against related viruses, as seen in the phenomenon of cross-protection. The mechanisms thought to be involved are discussed. Localized infections also produce protection but against a wide range of unrelated pathogens, including bacteria and fungi. This involves an active response by the infected plants, switching on a number of different resistance mechanisms for the different types of pathogens. Research on these plant-coded, induced proteins thought to be involved in resistance is discussed.