In a broad sense, bacterial diseases include infections produced by ordinary cocci and bacilli and also by mycobacteria, rickettsiae, and spirochetes. This heterogeneous group of microorganisms can produce tissue damage in various ways and elicit different kinds of tissue reactions. Bacteria cause cell injury by secreting exotoxins or by releasing endotoxins from their cell walls when they are destroyed. Tissue reactions to bacteria may be acute and nonspecific, dominated by polymorphonuclear leukocytes, or chronic and specific, such as the formation of granulomas. Lymph nodes that drain tissues infected by bacteria may be subjected to similar damage that results in various combinations of suppurative, necrotic, proliferative, and sclerosing lesions.