A disease cluster is the occurrence of a greater than expected number of cases of a particular disease within a group of people, a geographic area, or a period of time. Clusters of various diseases have concerned scientists for centuries. Some recent disease clusters include the outbreak of Legionnaire's disease in the 1970s from contaminated water in air conditioning ducts, the initial cases of a rare type of pneumonia among homosexual men in the early 1980s that led to the identification of human immunodeficiency virus/acquired immune deficiency syndrome (HIV/AIDS), and periodic outbreaks of food poisoning caused by eating food contaminated with bacteria.
Cancer clusters may be suspected when people report that several family members, friends, neighbors, or co-workers have been diagnosed with cancer. In the 1960s, one of the best-known cancer clusters emerged, involving many cases of mesothelioma, a rare cancer of the lining of the chest and abdomen. Researchers traced the development of mesothelioma to exposure to a fibrous mineral called asbestos. Exposure to asbestos, which was used heavily in ship building during World War II and has also been used in manufacturing many industrial and consumer products, is the only known risk factor for mesothelioma.