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Here is an essay on ‘Viruses and Cancer’ which states – How viruses and other infectious agents can cause cancer.
The ability of chemicals and radiation to cause cancer was widely recognized by the early 1900s, but the possibility that infectious agents might also be involved was not seriously considered at that time because cancer does not generally behave as if it were contagious.
However, in 1911 Peyton Rous carried out a series of studies on sick chickens brought to him by local farmers that showed for the first time that cancer can behave like an infectious disease (Figure 12). The chickens brought to Rous turned out to have malignant tumors of connective tissue origin, or sarcomas.
To investigate the origin of the tumors, Rous ground up samples of tumor tissue obtained from the sick chickens and passed the material through a filter whose pores were so small that not even bacterial cells could pass through it. When he injected this clear, cell-free extract into healthy chickens, Rous found that these chickens also developed sarcomas. Since no cancer cells had been injected into the healthy chickens, Rous concluded that the sarcomas were transmitted by an infectious agent that was smaller than a bacterial cell. This study represented the first demonstration of the existence of an oncogenic virus—that is, a virus that causes cancer.
Despite the clarity of the Rous experiments, his conclusions were initially greeted with skepticism. At one point he was told that “this can’t be cancer because you know its cause,” and it took many years before the existence of cancer-causing viruses came to be widely accepted. Convincing proof required the isolation and characterization of a large number of viruses that trigger different kinds of cancer.
Scientists began to discover such viruses in the 1930s, and the list of oncogenic viruses today has grown to include dozens of examples that cause cancer in animals. Several viruses have also been implicated in the development of human cancers.
Included in this group are the Epstein-Barr virus associated with Burkitt’s lymphoma, human papillomaviruses associated with cervical cancer, and the hepatitis B and hepatitis C viruses associated with liver cancer. As a group, these viruses are responsible for about 10% of all cancers worldwide.
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Unlike chemicals and radiation, whose carcinogenic effects are based on their ability to cause random DNA damage, some cancer-inducing viruses act by introducing specific genes into target cells. This property has made cancer viruses important to the field of cancer biology not just for the few kinds of human cancer they cause but also as research tools for identifying the types of genes that underlie the development of cancer.
In recognition of the numerous contributions to cancer biology that have emerged from the study of cancer viruses, Rous was finally awarded a Nobel Prize in Medicine in 1966. Although Nobel Prizes were originally intended to recognize recent scientific achievements, an 87-year-old Rous received the prize more than 50 years after his pioneering discovery of the virus, now called the Rous sarcoma virus that causes cancer in chickens.
Sometimes it takes science a long time to recognize the achievements of the pioneers who open up new fields of investigation.
