A virophage is a subviral agent that uses a virus' reproductive machinery to propagate itself, and in doing so, inhibits the reproduction of the host virus. As of 2012, there are only three known virophages, called the sputnik virophage, the mavirus virophage, and Organic Lake Virophage (OLV). These virophages have only been observed in low order life forms like amoebae, but researchers hope to discover virophages for viruses that affect humans, so that they could possibly be co-opted as a medical treatment.
Part of the reason virophages work is because of the structure of a virus. Viruses are infectious and they contain genetic material, but they are not usually described as being “alive” in the sense that bacteria and other infectious agents are. All viruses, whether simple or complex, are covered by a protein envelope known as a capsid; researchers often try to develop medications which will target the proteins in the capsid in an attempt to destroy viruses in their subjects.
In the case of a virophage, the agent can only exist in an organism where another virus exists. In the case of the sputnik virophage, it can only exist in amoebae that contain the Acanthamoeba polyphaga mimivirus, also called the mamavirus. The sputnik virophage then uses the mamavirus' reproductive equipment, which inhibits the reproduction of the mamavirus and makes it produce abnormal forms of itself that can't survive — for instance, the mamavirus may produce an abnormal capsid. One study found that the sputnik virophage reduces the mamavirus' reproduction by 70 percent.
Though virophages have only been found in low order life forms so far, researchers hope to find ones that affect viruses that cause human illness. If such virophages were found, they could possibly be used as part of medical treatments for viral illnesses, but this type of treatment is still entirely theoretical.
Virophages are often compared to bacteriophages, viruses which have evolved to prey on bacteria. Much more is known about bacteriophages; bacteria also represent a natural pool of potential resources for viruses, so it makes sense that viruses would adapt to take advantage of them. Bacteriophages also have potential when it comes to the treatment of disease; they could, for example, be used to assault drug-resistant bacteria.