The streptococci are a widely distributed group of organisms which can be important pathogens or commensals of human beings and other animals; some species may be found as saprophytes in the natural environment. Streptococci are frequently found on the mucous membranes of the oral, respiratory, alimentary, and urogenital tracts and skin of normal people and many other animals. Streptococci are usually aerobic or facultative anaerobes, but some strict anaerobes are included in the genus Streptococcus. Streptococcus suis is essentially an organism confined to pigs, and it has been estimated that all pigs over the age of 6 weeks harbor the organism in their palatine tonsils. Many species and types of streptococci affect different species of animals, including human beings. Some of these have been recorded in both animals and human beings and without further study might be considered zoonotic.