Oxidation is a chemical reaction that transfers electrons from a substance to an oxidizing agent. Oxidation reactions produce free radicals, including reactive oxygen species and reactive nitrogen species. These oxidants are produced during metabolism and may be substantially increased by aerobic exercise, stress, tissue injury, infection and detoxification of many compounds. Stress may precede an infectious episode in animals by decreasing antioxidants needed later by an active immune response. Antioxidants terminate the cell-damaging chain reactions caused by oxidation by removing free-radical intermediates. When antioxidants are deficient, free radical and non-free radical oxidants can produce destructve effects in animal tissues. When free-radical production is greater than the ability of the animal to quickly detoxify these products, oxidative stress occurs.