What are interferons? From Wikipedia:
Interferons (IFNs) are natural proteins produced by the cells of the immune system of most vertebrates in response to challenges by foreign agents such as viruses, parasites and tumor cells. Interferons belong to the large class of glycoproteins known as cytokines. Interferons are produced by a wide variety of cells in response to the presence of double-stranded RNA, a key indicator of viral infection. Interferons assist the immune response by inhibiting viral replication within host cells, activating natural killer cells and macrophages, increasing antigen presentation to lymphocytes, and inducing the resistance of host cells to viral infection.
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How does interferon prevent viral replication? Its binding to the plasma membrane triggers the synthesis of serveral enzymes by the cell. If the cell is infected or eventually becomes infected, these enzymes block the synthesis of proteins the virus requires for replication. It must be reemphasized that interferon is not specific. Many, but not all, viruses induce interferon synthesis, and interferon in turn can inhibit the multiplication of many kinds of viruses.