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  3. What Causes Cancer? The question "What causes cancer?" has been asked for centuries. The short answer is: we still don’t kno

What Causes Cancer? The question "What causes cancer?" has been asked for centuries. The short answer is: we still don’t kno

admin2010-05-26  23
问题                     What Causes Cancer?
    The question "What causes cancer?" has been asked for centuries. The short answer is: we still don’t know. But a tremendous amount has added to our understanding in recent decades. One major change in the way researchers view cancer is that they now rarely think in terms of a single unitary cause. Rather, it appears that most cancers are caused by the interaction of several factors. In ninny cases, some or all of three factors may be involved: (1) viruses, (2) individual susceptibility, and (3) environmental irritants(刺激物). Another major change in our contemporary view of cancer is that it is now regarded as a premiere example of an environmental disease--that is, a disease in which environmental factors are often crucial to the disease process.
    VIRUSES
    A number of viruses have been identified that induce cancer in laboratory animals. What about humans?  As of the late 1970s, no virus had been shown conclusively to cause cancer in people. However, the scientific consensus was that it was probably only a matter of time before such a connection was proven.
    Cancer, however, is not a communicable disease, like a cold or influenza. It is not "catching" in the ordinary sense of a word, and the mere presence of the suspect virus is apparently not enough, by it self, to produce the disease. It appears that the process may work something like this: A carcinogenic virus enters a cell and insinuates itself amidst the cell’s genetic material. (Viruses and genes are structurally similar. ) The virus may then lie dormant(休眠的) for years, until it is triggered into action by some kind of environmental irritant, such as pollution or radiation. At that point, the reactivated virus causes changes in the genes, altering the host cell permanently and also altering all the cells produced by the division of the original cell. The genetic program of these new—and now cancerous-cell calls for tile unrestrained  growth, dedifferentiation(分化), anaphase, and metastasis(转移)discussed earlier.
    INDIVIDUAL SUSCEPIBILITY
    Some people may be more prone to develop cancers than others, for several possible reasons.
    Heredity is one. Resistance to cancer, or susceptibility to it, is a quality for which laboratory animals can be bred. So, presumably, heredity may play some role in humans as well. Statistical evidence, however, shows only a minor tendency for cancers to run in families. Indeed, for most cancers no such tendency can be demonstrated. Cancers of the breast, lung, thyroid, colon, and rectum do seem to cluster in families, at least to some degree. If your family has a history involving one of these cancers, you and your physician may want to be especially vigilant for signs of such cancer during your periodic physical examinations.
    Some researchers have proposed that personality characteristics may render some people more vulnerable  to cancer. According to this theory, people are more likely to develop cancer, and more likely to die from it, if they have rigid, authoritarian personality and suppressed inner conflicts about sexual and aggressive feelings.  There have been many studies attempting to test this theory, and the results are thought-provoking. In one study, for example, doctors at the University of Rochester were able to predict with seventy-five percent accuracy whether women entering the hospital for a biopsy would have cancer or not, based on psychiatric interviews. However, this and many other studies linking cancer and psychological factors were performed on small samples. Well-controlled, large-scale, long-term studies are needed before the theory can be regarded as well established.
    A third factor that might influence your susceptibility to cancer is the state of your immune system. When a person’s immune system is weak, he or she has a heightened chance of developing cancer. The immune system can be weakened by a hereditary defect, by exposure to radiation, or by the action of immunosuppressive drags (given, for example, to people who have organ transplants).
    ENVIRONMENTAL IRRITANTS
    You may have been surprised to learn that environmental factors play a role in funning sixty percent of cancer. You may also wonder: how do we know?
    We know, in large part, by studying the geographical patterns of cancer,! Lung cancer, for example, is common in the United States and Britain, rare in Africa and most of Asia. Colon and rectal cancers are common in the West, but rare in developing countries. Rates of various cancers also vary dramatically in various regions of the United States.
    You might think that genetic or ethnic factors could account for these geographical differences. But careful studies of people who move from one country to another showed that immigrant populations quickly
lost the patterns in their country of origin and developed the cancer patterns typical of their new environment.
    If more evidence is needed that environmental irritants play a major role in the development of cancer, it is abundantly supplied by the relationship between certain occupation and certain types of cancer. People who work with asbestos have high rates of lung cancer; so do uranium miners and people who tend coke(焦炭) ovens at steel mills. People who work with aniline dyes or in the rubber industry have elevated rates of bladder cancer. Being exposed to substances such as metals, dust, chemicals, or pesticides at work can increase the risk of cancer. Asbestos, nickel, cadmium, uranium, radon, vinyl chloride, benzidine, and benzene are well-known examples of carcinogens(致癌物) in the workplace. These may act alone or along with another carcinogen, such as cigarette smoke. For example, inhaling asbestos fibers increases the risk of lung diseases, including cancer, and the cancer risk is especially high for asbestos workers who smoke. It is important to follow work and safety rules to avoid contact with dangerous materials.
    Exposure to large doses of radiation from medical X-rays can increase the risk of cancer. X-rays used for diagnosis expose you to very little radiation and the benefits nearly always outweigh the risks. However, repeated exposure can be harmful, so it is a good idea to talk with your doctor or dentist about the need for each X-ray and ask about the use of shields to protect other parts of your body. Before 1950, X-rays were used to treat noncancerous conditions (such as an enlarged thymus, enlarged tonsils and adenoids, ringworm of the scalp, and acne) in children and young adults. People who have received radiation to the head and neck have a higher-than-average risk of developing thyroid cancer years later. People with a history of such treatments should report it to their doctor and should have a careful exam of the neck every 1 or 2 years.
    In some of these instances, it is possible to pinpoint a particular environmental irritant, such as asbestos or polyvinyl chloride, which is involved in the process of cancer development. In the population at large, however, it is a guessing game trying to ascertain which pollutants and other substances to which we are exposed, or which combinations of these substances contribute to cancer. Are the major offenders in the water we drink? In the meat we eat? In the air we breathe? For the most part, we simply don’t know. There are so many variables that analysis is extremely difficult.
The intent of the author in writing this passage is to______readers.
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解析 由文章的大标题可以看出，它主要是让读者了解有关癌症的知识。
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