Osteoporosis is a disease of the bones that happens when someone loses too much bone, makes too little bone or both. As bones become weaker, they become more likely to break. Osteoporosis-related fractures most commonly occur in the hip, wrist or spine. Osteoporosis is a common disease, affecting about nine million Americans and, most commonly, older women. Risk factors include low bone density, aging, being small and thin, family history, certain medicines and being a white or Asian woman. Preventive measures include keeping bones strong by eating a diet rich in calcium and vitamin D, exercising, not smoking and taking certain medications.