The histological appearance of the lesions present in the tissues of patients with primary or recurrent infections is indistinguishable. The specific cellular changes are the same in all infected tissues and have been described in detail in the section on Cytopathology. In addition to the cytological alterations common to all tissues, cytopathic changes will occur that are characteristic for certain tissues. Thus, in the skin, the mucous membrane, the cornea, and the chorioallantois of the chick embryo, there is a proliferation of the infected epithelial cells which thicken, become edematous, and eventually slough away, leaving an acute inflammatory, nonspecific ulceration. The changes in the skin are characterized by vesicle formation in the malpighian layer cells (Scott et al., 1950; Scott and Tokumaru, 1965). The vesicles are filled with exudate containing infected epithelial cells, white blood cells, and necrotic cell debris. An inflammatory reaction occurs in the corium, with dilated blood vessels and cellular exudate. The mucosal lesion is similar. In generalized infections, the viscera are affected, including the liver and adrenals which show coagulation necrosis with cells surrounding the area presenting characteristic intranuclear inclusions. In the brain, the specific cytopathic changes cause the ultimate death of the infected cells, developing into areas of disintegrated neurons, as well as centers of necrosis with a surrounding inflammatory reaction.