Several biological, environmental, and behavioral patterns may contribute to increase the incidence of disease or death in a given population, and such deleterious patterns are generically termed risk factors. For instance, several recent studies points to tobacco addiction as the leading cause of premature death in the United States, accounting for approximately 500,000 deaths (i.e., about 20% of all deaths) every year. Smoking, therefore, is a serious risk behavioral factor contributing to the development of coronary disease, cancer, stroke, pulmonary emphysema, and other respiratory and circulatory disorders.

Biological risk factors may be either an inherited gene mutation that increases the predisposition to one or more diseases, such as cancer, diabetes mellitus, or familial hypercholesterolemia; or they may be an infection by viruses or bacteria that induce DNA mutations in the cells of the host, such as hepatitis virus (liver necrosis or liver cancer), Helicobacter pylori (gastric cancer...