Preventive medicine is based on epidemiologic studies that have identified risk factors that predispose to disease. There is good evidence that, in many areas, early and timely preventive strategies and interventions can reduce subsequent morbidity and mortality. Primary prevention refers to interventions designed to reduce the risk of disease onset. An example, is a vaccine to prevent poliomyelitis. Secondary prevention includes efforts to improve outcomes in those with preclinical disease; for example, a mammogram or breast self-examination to screen for breast cancer. Tertiary prevention encompases efforts to prevent the progression of symptomatic conditions through systematic identification, treatment, and rehabilitation. A good example is rehabilitation after a stroke, to improve functional status and the quality of life.