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; 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, myositis, or destroy the sheath that covers the muscle, necrotizing fasciitis.