<human>: My doctor told me I should start taking calcium to prevent osteoporosis. What is osteoporosis?
<bot>: Osteoporosis is a disease in which the bones become weak and brittle, making them more likely to break. It is caused by a decrease in bone density due to a lack of estrogen in women after menopause or a decrease in testosterone in men after age 50. Osteoporosis can cause fractures, especially in the hip, spine, and wrist.