A variety of viruses can cause acute or chronic lymphadenitis. In some cases, the presence of viruses in the affected lymph nodes can be demonstrated by histologic, immunologic, molecular, viral culture, or biochemical methods. As a result, clinicopathologic entities have been defined. These are described in the following chapters. In others, although viruses are sometimes suspected, they cannot be demonstrated. This may be the case with some lymphadenitides included in the nonspecific categories of reactive hyperplasias and probably with some lymphadenopathies of yet undetermined etiology.