Necrotizing fasciitis: Severe bacterial infection of the fascia, the tissues that line and separate muscles, that causes extensive tissue death. Necrotizing fasciitis can be caused by several different types of bacteria, particularly by virulent strains of streptococcus and staphylococcus. The rapid spread and destruction of tissue occurs because of substances produced by the bacteria. Treatment involves the use of high-dose antibiotics and surgical removal of dead and infected tissue to help control the infection. Also known as flesh-eating bacteria.