Osteoporosis

Osteoporosis is a progressive debilitating bone disease where bone mass slowly disintegrates leaving the bones brittle and less able to bear loads. Fractures, including serious fractures to load bearing bones such as the hip, become common after even minor trauma, like a fall on a level surface. It's most common in women over the age of 65, but can affect males and younger people as well although it's far less common.
Osteoporosis is difficult to treat. Calcium supplements do seem to build bone mass in many patients, but the bones often remain weak. Women over the age of 40 are often advised to take calcium supplements to avoid the onset of the disease. However, lack of exercise (particularly those that build strength, such as running and weightlifting) has recently been tied to the onset of the disease.
Osteoporosis at Wikipedia
Osteoporosis at Mayo Clinic