Humans have been struggling with cancer for centuries but it has been only in the last 100 years that many of the root causes of cancer have been revealed.
There are a few genetically heritable conditions that are known to lead to a high probability of developing cancers in tissues such as skin or breast. However, the great majority of malignancies are found to harbor multiple genetic mutations which are present only in the tumor itself. These mutations are called “somatic” mutations because they are not found in the genes of the host’s sperm or eggs and therefore must have occurred in the afflicted tissue sometime during its conversion from healthy tissue to a cancer.
The events that induce or set the stage for most of these cancer-causing somatic mutations in humans are well known: 30 percent are linked to tobacco use, 35 percent to diet, about 15 percent to obesity, about 15 percent to infectious agents, and about 5 percent to radiation or chemicals.
Because many cancers have highly unstable genes, cancers are often characterized by multiple types of malignant tissue within the same tumor, and those arising in the same organ in different people may behave differently, all of which complicates the treatment of these illnesses. In the late 1800s, William Coley, a surgical oncologist, knowing that it had been previously observed that bacterial infections occasionally led to the remission of cancers, developed a mixture of bacteria with which to treat cancer patients. He then purposefully infected cancer patients with these bacteria in order to induce an immune reaction that, when successful, led to the regression of the tumor. However, more recent studies of our immune responses to cancer have revealed that these responses are at least as likely to promote the cancer as they are to cause its removal. Detrimental immune responses include the inactivation of natural tumor destroying enzymes and the promotion of the activity of genes that facilitate cancer growth.
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