The question the reader will ask at this point is ‘Given all this epidemiological study, do we know the causes of cancer?’ Broadly the answer is ‘yes’ in many circumstances and for many cancers, and the opportunities for prevention that this understanding generates are there to be taken. We do not always know how the factors that have been identified by the epidemiological studies discussed in this chapter link up to what is being learned in the laboratories of the molecular biologists. This connection is being made rapidly and will be increasingly clear by the end of the century. Epidemiology has been very successful in discovering or confirming which features of our lives in the Western world can be now identified as causes of cancer.
Simple infections do not cause cancer. Pneumonias and urinary infections, for instance, are usually caused by bacteria and there is no evidence that such infections predispose to cancer in any way. Animal cancers can be caused by viruses but human cancers are not usually caused by viruses. There are, however, some notable exceptions to this general statement. The virus described by Epstein and Barr (Epstein-Barr virus, EBV) probably causes a rare cancer of the lymph glands, particularly in Africa, and may cause cancer of the nasal passages among the Chinese. Hepatitis B virus infection, when chronic, probably contributes to the high incidence of liver cancers in the Far East, the evidence for this being a most convincing cohort study in Taiwan. Rare types of leukaemia, particularly in Japan and the Caribbean, have been linked to infection with a particular kind of virus (human T lymphotrophic virus type 1), which seems to be spread early in life but which may alio, like AIDS, be spread by sexual activity and drug abuse. AIDS infection predisposes patients to a number of cancers of a rare kind which may be very difficult indeed to treat. As indicated above, viruses are being investigated as a possible explanation for a link between cancer of the neck of the womb and multiple sexual partners. It should be emphasized that human cancer is not in any simple way an infectious disease, that patients with cancer do not require isolation and that people need not be concerned about sharing homes or workplaces with cancer patients.
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