Endogenous agents are substances that are in our body that can cause DNA damage directly or indirectly, for example, bile acids or reactive oxygen species. Exogenous agents are agents that are in our environment like cigarette smoke, radiation, or smog. Diet is an exogenous agent and consumption of certain foods, like processed meats, have been linked to a higher incidence of cancer while consumption of other foods, like fruits and vegetables, have been linked to a lower risk of developing a tumor.