Before modern medicine and technology, medical treatments were crude to say the least. For example, babies who lived in countries with little sun during the winter were actually given tanning treatments to prevent rickets. Sunlight, especially ultraviolet light, lets human skin cells convert vitamin D from an inactive to active state. In the absence of vitamin D, dietary calcium is not properly absorbed, resulting in hypocalcaemia, leading to skeletal and dental deformities and neuromuscular symptoms, e.g. hyperexcitability. Continue reading for more.