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 ual 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 sy-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 ca