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Streptococcus is a genus of gram-positive cocci, or bacteria, with a thick peptidoglycan layer in their cell walls (gram-positive) that appear under the microscope as chains of two or more spherical cells (cocci). The streptococci can grow in low concentrations of oxygen or without oxygen and are distinguished from bacteria in the genus Staphylococcus in the laboratory by the production of the enzyme catalase by staph species.

Pneumococcus (plural pneumococci), also known as Streptococcus pneumoniae, is a highly infectious bacterial microorganism that can cause a number of serious illnesses, some of which can be life-threatening. The bacterium is most commonly found in the human respiratory tract and can be transmitted from person to person through droplets dispersed in the air. More than ninety strains of pneumococcal bacteria exist, but only a few produce the most serious illnesses, such as pneumonia, sepsis, and meningitis.