Staphylococci are coagulase-negative (Staphylococcus epidermidis) or coagulase-positive (Staphylococcus aureus) gram-positive bacteria. Coagulase-negative staphylococci grow abundantly as normal flora on skin, but they can also cause boils, abscesses, and carbuncles. In the upper respiratory tract, they are usually nonpathogenic but can cause serious infections. Pathogenic strains of staphylococci are found in many adult carriers—usually on the nasal mucosa, axilla, or groin. Sometimes, carriers shed staphylococci, infecting themselves or other susceptible people. Coagulase-positive staphylococci tend to form pus; they cause many types of infections.

For specific information on different types of staphylococcal infections, see Comparing staphylococcal infections.