The concept that viruses cause human cancer dates back to the first decade of the 20th century, when experiments on animals showed that tumors could be induced in chickens by an agent that could pass through a filter. This pioneering work by Francis Peyton Rous in the United States was recognized more than 50 years later with the 1972 Nobel Prize that he shared with Howard Temin and David Baltimore, who characterized the molecular biology of retroviruses, first detected in Rous's experiments. Over the last decade, the new tools of molecular biology have led to profound discoveries about the role of viruses in human cancer and the mechanisms of disease causation. Viruses are a type of infectious agent that must invade living cells in order to reproduce. The two major types of viruses have either DNA or RNA as their genetic material.
Viruses often invade cells by attaching to receptors on the surface of the target cell.
Once inside the cell, they often integrate their genetic material into that of the host and alter the cell in ways that predispose to cancer through a variety of mechanisms. In addition, genes called oncogenes, first discovered as part of the genetic material of acutely transforming retroviruses but now recognized as part of the normal genetic makeup of the cell, have been identified as critical factors in the oncogenic process. A characteristic feature of these tumors is the monoclonal integration of the viral genome in the tumor tissue. However, viral genes have the capacity to turn on genes of the host cell, promoting cell growth that may lead to uncontrolled cell proliferation. The HTLV-I virus also causes a pediatric immunodeficiency syndrome called infective dermatitis and adult autoimmune diseases such as polymyositis and arthritis.