Gram-positive and gram-negative cocci are among the most significant infectious agents of humans as well as prevalent members of the normal flora of the skin, oral cavity, and intestine. Because infections with these bacteria tend to stimulate pus formation, they are often referred to collectively as the pyogenic cocci. The most common infectious species in this group belong to four genera: Staphylococcus, Streptococcus, Enterococcus, and Neisseria.