Selenium's principal function is to inhibit the oxidation of lipids(fats) as a  component of the enzyme glutathione peroxidase. It is a vital anti-oxidant, especially combined with vitamin E. It protects the immune system by preventing the formation of free radicals, that can damage the body. It plays a vital role in regulating the effects of thyroid hormone on fat metabolism. 

Selenium has also been found to function as a preventative against the formation of certain types of tumors. One study found that men who took 200 micrograms of selenium daily over a ten-year period had roughly half the risk of developing lung, prostate, and colorectal cancer as compared with men who did not. 

Selenium and vitamin E act synergistically to aid in the production of  antibodies and to help maintain a healthy heart and liver. This trace element is needed for pancreatic function and tissue elasticity. When combined with vitamin E and zinc, it may also provide relief from an enlarged prostate. Selenium supplementation the has been found to protect the liver in people with alcoholic cirrhosis. Studies conducted by the University of Miami indicate that taking supplemental selenium may enhance the survival of people with AIDS, by increasing  both red and white blood cell counts. It has shown promise in the treatment of arthritis, cardiovascular disease, male infertility, cataracts, AIDS, and high blood pressure.   

Selenium deficiency has been linked to cancer and heart disease. It has also  been associated with exhaustion, growth impairment, high cholesterol levels, infections, liver impairment, pancreatic insufficiency, and sterility. There is  some thought that selenium deficiency might be linked to a host of outbreaks, from new strains of influenza to Ebola, wrought by the rapidly  mutating virus's  interaction with selenium-deficient hosts in places like Africa and China where there is little or no selenium in the soil.