Of course, this is not to say that we can’t identify some of these factors. The immune system can be weakened by other illnesses, poor nutrition, some drugs, some infections, stress, old age, cancer itself and some rare inherited disorders. Very active cancer cells can just develop by chance, but the more mutations that are occurring, the greater chance that one will lead to a cancer. The number of mutations is increased by exposure to radiation, some drugs, sunlight (skin cells only), many chemicals, and some types of infection. Exposure to any one of these factors does not necessarily lead to cancer. Thus, although we know that there are cancer-producing chemicals in cigarette smoke, not everyone who smokes gets cancer. Some smokers are lucky enough either to miss out on any mutations which lead to cancer or to have an effective immune system which prevents any cancers from developing. It usually takes more than one factor to produce cancer—unfortunately we have by no means identified them all yet.
Let me reassure you on two particular factors which worry many people with cancer.
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