Selenium, a metalloid mineral micronutrient,is an essential component for the adequate and healthy life of human, animals, and some other microorganisms.Selenium has antioxidative and oxidative effects on living organisms.Selenium deficiency has been associated with impaired function of the immune system.Selenium deficiency appears to enhance the virulence or progression of some viral infections.Selenium deficiency results in decreased activity of glutathione peroxidase, increasing oxidative damage and the likelihood of mutations in the viral genome.The safe range of selenium intake is relatively small and excess selenium intake can readily result in toxicity.This review summarizes the updates of benefitial and adverse effects of selenium on health.