If the evacuation, for example via the lymph capillaries, is overloaded, pus may collect in the tissue. Pus mainly consists of white blood corpuscles (leucocytes), residual tissue and pathogens. A diffuse swelling, an infiltrate, often exists before pus is formed. The formation of pus in a natural body cavity (e.g., sinuses) is called an empyema. If pus spreads in the tissue without immunological limitation, it creates a phlegmon, mainly caused by streptococci. But pus frequently collects into an abscess, a cavity created by necrosis and pus-filled melting of the affected tissue. In contrast to a phlegmon, this pus collection within the tissue is surrounded by a membrane consisting of granulation tissue, which clearly isolates it from the environment.