Currently, in a broad sense, much is known about the causes. Diet, radiation, smoking, viruses, environmental pollutants and workplace carcinogens all can play a role. Some of these links are well-understood, others hardly at all. .
Smoking, for example, is linked to lung cancer, and sunlight is considered a cause of skin cancer. Epidemiological studies also strongly link benzene to leukemia and asbestos to lung tumors. "But for many other chemicals, our knowledge is much rougher," says Philip Landrigan, director of environmental medicine at the Mt. Sinai School of Medicine in New York City. "And for diet, our knowledge is much less specific." (Advances in Cancer Research) .
However, this is still not the whole picture. At the most basic level, scientists readily agree that cancer is a genetic disease. It results from a series of mutations in some of the 100,000 or so genes that are present in every cell. It takes years, usually decades, for enough mutations to accumulate and cause a cell to become cancerous. The cancer spreads as that cell divides and passes on its damaged DNA to daughter cells. Scientists have identified dozens of genes, that, when mutated, are linked to cancer. They classify them into three types: oncogenes, which when normal signal cells to divide conventionally but when damaged can cause them to multiply uncontrollably; tumor suppressor genes, which act as the brakes on cell division but when mutated lose that ability; and mutator genes, which protect other genes from developing mutations until they themselves mutate. (Genes and the Biology of Cancer).
What scientists don't fully understand is what causes people who have inherited normal genes - the majority of the population - to develop gene defects that eventually lead to cancer. The answer is complicated by the nature of cancer. Cancer, in a sense, is not one disease but scores of different diseases that share the common feature of uncontrolled cell growth.