Osteoporosis occurs when bones become weak and are more likely to fracture. Common causes include aging, lack of calcium and vitamin D, insufficient exercise, smoking, excessive alcohol consumption, and certain diseases and medications. To prevent osteoporosis, you can: eat foods rich in calcium and vitamin D, like milk and fish; engage in regular weight-bearing exercises, such as walking and running; avoid smoking and limit alcohol intake; and regularly check bone density, especially for the elderly and those with a family history.