Reactive oxygen and nitrogen metabolites play a complex role in many diseases and in metabolic regulation. Because viruses replicate in living cells, such metabolites influence the growth of viruses in addition to serving as a host defense mechanism. Low levels of reactive oxygen species (ROS) play a role in mitogenic activation, and the early phase of lytic and nonlytic virus infection indeed resembles that of mitogenic cell activation. In addition to these subtle cell-activating effects shared by many viruses, influenza and paramyxoviruses activate a respiratory burst in phagocytic cells. These viruses are toxic when injected in animals. Cells lavaged from the lungs of mice infected with influenza virus are primed for enhanced superoxide generation. Moreover, xanthine oxidase is enhanced and the buffering capacity of small molecular antioxidants is decreased in the lungs, suggesting that infection leads to oxidative stress. The wide array of cytokines produced in the lungs during influenza could contribute to the systemic effects of influenza. Oxidative stress has also been shown in human immunodeficiency virus (HIV) infection in humans. Via activation of NF kappa B, ROS may activate viral replication, but oxidants are believed to contribute also to the loss of CD4 T cells by apoptosis. Antioxidants, together with agents interfering with the harmful effects of cytokines and lipid mediators, may have a role in the treatment of viral diseases. Such agents could not only alleviate disease symptoms but also decrease the long-term effects of chronic oxidative stress, which have been linked to the development of cancer in some viral infections.