Osteoporosis. Osteoporosis is a disease of the bones that causes them to be fragile, resulting in fractures. The bones actually become thinner and less dense. This is either due to the body not forming enough new bone, or too much bone being reabsorbed into the body, or both. Osteoporosis is more common in women than men and tends to develop after menopause. Cause: There are many causes of osteoporosis.