Rickets (or rachitis) is a bone disorder primarily caused by a deficiency of vitamin D during a child's prime bone-growing years. The lack of vitamin D seriously affects the body's ability to absorb and use calcium, the mineral most necessary for the proper growth, development, and hardening of bone. Without sufficient calcium, the child's bones grow more slowly, become weaker, and tend to bend more easily. Because the weight-bearing bones are most seriously affected, the child's legs and pelvis may be misshapen--sometimes to a crippling degree.

Lack of sunlight also appears to contribute to the severity of rickets because, normally, the body can produce its own vitamin D from fatty precursor substances--but only in the presence of sunlight.

Although known for centuries, rickets became an increasingly serious problem in the industrial age, reaching its peak in the nineteenth century and the early part of the twentieth century. As...