The exact cause of cancer is not known. Most cancers result from permanent damage to genes or from mutations, which occur either due to internal factors, such as hormones, immune conditions, metabolism, and the digestion of nutrients within cells, or by exposure to environmental or external factors. A chemical or other environmental agent that produces cancer is called a carcinogen.
Overall, environmental factors, defined broadly to include tobacco use, diet, infectious diseases, chemicals, and radiation, are believed to cause between 75 and 80 percent of all cancer cases in the United States. Tobacco use, including cigarettes, cigars, chewing tobacco, and snuff, can cause cancers of the lung, mouth, throat, larynx, bladder, kidney, esophagus, and pancreas. Smoking alone causes one-third of all cancer deaths in the United States. Heavy consumption of alcohol has also been shown to increase the risk of developing cancer of the mouth, pharynx, larynx, esophagus, liver, and breast.
Overweight and obesity are associated with increased risk of cancers of the breast, colon, endometrium, esophagus, kidney, and gallbladder. The following chemicals have been found to cause cancer: coal tars and their derivatives, such as benzene; some hydrocarbons; aniline, a substance used to make dyes; and asbestos. Radiation from a variety of sources, including the ultraviolet light from the sun, is known to lead to skin cancer.
Several infectious agents have also been implicated in cancer. Evidence suggests that chronic viral infections are associated with up to one-fifth of all cancers. These include hepatitis B virus (HBV), which can lead to cancer of the liver; the Epstein-Barr virus, a type of herpes virus that causes infectious mononucleosis and has been associated with Hodgkin's disease, non-Hodgkin's lymphomas, and nasopharyngeal cancer; the human immunodeficiency virus (HIV), which is associated with an increased risk of developing several cancers, especially Kaposi's sarcoma and non-Hodgkin's gene: DNA sequence that codes for proteins, and thus controls inheritance hormone: molecules produced by one set of cells that influence the function of another set of cells metabolism: the sum total of reactions in a cell or an organism nutrient: dietary substance necessary for health infectious diseases: diseases caused by viruses, bacteria, fungi, or protozoa, which replicate inside the body overweight: weight above the accepted norm based on height, sex, and age obesity: the condition of being overweight, according to established norms based on sex, age, and height chronic: over a long period
Tobacco use is a major cause of lung, lip, mouth, larynx, and throat cancer, and is a contributing cause of many other cancers. In India, where this photo was taken, the prevalence of tobacco use among students approaches 60 percent in some states. [© AFP/Corbis. Reproduced by permission.]
virus: noncellular infectious agent that requires a host cell to reproduce
DNA: deoxyribonucleic acid; the molecule that makes up genes, and is therefore responsible for heredity lymph node: pocket within the lymph system in which white blood cells reside genetic: inherited or related to the genes
When cells in some area of the body divide without control, these cells accumulate and form lumps. A tumor-, or neoplasm, is an abnormal lump or mass of tissue that may compress, invade, and destroy normal tissue. Tumors may be benign or malignant. Cancer is a malignant neoplasm, though not all tumors are malignant. A noncancerous growth is called a benign tumor. Benign tumors do not metastasize and, with very rare exceptions, are not life threatening.
lymphoma; and human papilloma viruses (HPV), which have been proven to cause cervical cancer and have also been associated with cancers of the vagina, vulva, penis, and colon. The bacterium Helicobacter pylori has been linked to stomach cancer.
About 5 to 10 percent of cancers are hereditary, in that a faulty gene or damaged DNA that has been inherited predisposes a person to be at a very high risk of developing a particular cancer. Two genes, BRCA1 and BRCA2, have been found to cause some breast cancers. Other genes have been discovered that are associated with some cancers that run in families, such as cancers of the colon, rectum, kidney, ovary, esophagus, lymph nodes, skin melanoma, and pancreas.
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