Interferons (IFNs) are secretory proteins produced by virus-infected cells. The name interferon refers to their ability to interfere with viral replication. However, IFNs induce several pleiotropic responses, including antiviral, antitumor, immunomodulatory, antiparasitic, and antiproliferative activities. In addition to viral infection, double-stranded RNA (dsRNA), fungal cell wall products, and other cytokines also induce the production of IFNs. dsRNA may be a physiologically relevant regulator of IFN synthesis because it is thought to be an intermediate product of viral infection.