Viruses pass into the environment from clinically ill or carrier hosts; although they do not replicate outside living animals or people, they are maintained and transported to susceptible hosts. Population concentrations and movement, both animal and human, have been steadily increasing in this century, enhancing transmission of respiratory and enteric viruses and compounding the difficulty of preventing environmental transmission. Studies on environmental survival factors of viruses have been most definitive for polioviruses, foot and mouth disease viruses and Aujeszky's disease virus. In addition, heat resistance studies have been reported on adenoviruses, African swine fever virus and the Norwalk virus. Resistance to disinfectants has been studied for many viruses, including picornaviruses, papovaviruses, reoviruses and retroviruses. Survival of viruses in and on a variety of fomites has been studied for influenza viruses, paramyxoviruses, poxviruses and retroviruses. The subacute spongiform encephalopathy agents, under extensive current studies, are being found to have incredible stability in the environment.