Antivirals are substances other than a virus or virus containing vaccine or specific antibody which can produce either a protective or therapeutic effect to the clear detectable advantage of the virus infected host. The search for antiviral agents began in earnest in the 1950s but this was directed mainly by chance, with little or no scientific basis. It had a turning point in 1964 with a number of narrow spectrum agents whose values have been more difficult to establish. A lot of success has been achieved in the screening of plants for antibacterial, antifungal and antiviral actions. The use of plants or plant products, traditionally, as antiviral agents is relatively wider than their use in modern medicine. Some antiviral substances have so far been isolated from higher plants, algae and lichens. Suitable methods for evaluating antiviral properties of plants and their extracts include use of animal models, animal protection studies, egg inoculation studies and cell culture methods.