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A species of gram-positive, coccoid bacteria isolated from skin lesions, blood, inflammatory exudates, and the upper respiratory tract of humans. It is a group A hemolytic Streptococcus that can cause SCARLET FEVER and RHEUMATIC FEVER. Unlike the docile strains that cause strep throat, however, the virulent strains penetrate deep into the body, with catastrophic results. It has been demonstrated that invasive streptococcus A infections can trigger a toxic shock syndrome, chew up muscle (MYOSITIS), or destroy the sheath that covers the muscle (FASCIITIS, NECROTIZING).