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Epidemiologists study the cause, spread, and control of diseases affecting groups of people or communities. They use statistics, research, and field investigations to try to connect incidences of a disease with characteristics of populations and communities. Some epidemiologists focus on infectious diseases, which are caused by bacteria and viruses and include HIV, chicken pox, rabies, and meningitis. Others focus on noninfectious diseases including heart disease, lung cancer, breast cancer, and ulcers. Approximately 4,850 epidemiologists are employed in the United States.