When your body doesn't get enough Vitamin D a rare disease that you can get is Rickets from lack of Vitamin D in your body.
Rickets is a disease that causes a persons bones to become soft and bend which is more common in African American Children and Infants.
For Adults they can get what is called Osteomalacia which causes muscle weakness, bone pain and weak bones.
You can increase the amount of Vitamin D by increasing your exposure to sunlight and eating a proper diet.