The evidence relating the interferon system to the infectious process has been examined. The available evidence supports the view that the interferon system is an important component of the body's nonimmune defenses, which are probably the major causes of recovery from already established virus infections of body tissues. The interferon system can also serve to limit virus spread through the bloodstream. Factors which may influence the interferon system and thereby influence virus infection have been considered. Finally, evidence is presented which indicates that the interferon system is one of the determinants of virulence of certain viruses and is one of the determinants of some persistent virus infections.