Group A Streptococci are a species of gram-positive bacteria responsible for causing a number of pyogenic (pus-producing) infections including impetigo, scarlet fever and pneumonia. Further fatal complications arising from infection include the development of meningitis and sepsis.

Although all ages are susceptible to infection, group A streptococci primarily affect school-aged children accounting for 15-30% of pharyngitis cases. It has been estimated that up to 20% of asymptomatic school-aged children may be carriers of group A streptococci during the winter months. Generally, infections are treated easily with penicillin. Group A streptococci are also responsible for toxic shock syndrome (TSS) and necrotizing fasciitis, a rare 'flesh-eating' disease which in severe cases causes extensive limb damage, necessitating amputation. Each Streptococcus is 1 micrometer in diameter.