This chapter describes and illustrates the different lesions observed in central nervous system (CNS) infections. A wide variety of pathogenic infectious organisms may affect the CNS. They can be classified as pathogenic (causing diseases in every individual) or opportunistic (only affecting patients with impaired immune defenses). Bacteria may cause pyogenic infections or so-called specific infections in which the morphology of the lesions suggest a specific agent. Mycoses and parasitic infections were relatively uncommon and restricted to certain countries; however, due many factors in the modern world, including, for example, immunodeficiency of various causes and intercontinental travel, they are now more frequently encountered. Viral infections of the CNS cause lesions due to immunoallergic reactions or specific encephalitides. In AIDS, infection by HIV causes a unique encephalitis and immunodeficiency with a wide range of secondary opportunistic infections.