The widely held notion, common in clinical medicine, that the elderly are more susceptible to infection is discussed critically. Implied but unstated in this sentiment is a premise that host defense mechanisms are less effective in the elderly, or that immune surveillance becomes defective with aging. No clear evidence exists to support these concepts; indeed, available information points to a normally functioning immune system in the elderly. An increased incidence of morbidity and mortality resulting from infection in the elderly does exist, however, and it is suggested that this stems from many of the functional and anatomical deficits which accompany the aging process and therefore predispose the elderly to infection. Examples of these degenerative problems are pulmonary hypoventilation, bronchopulmonary aspiration, immobility, and urinary retention. These conditions can predispose the elderly to infection by common extracellular microorganisms that are the normal flora of the mucosal and skin surfaces of the body. More precise information on the nature of injuries caused by these organisms in the elderly and on the causes of the resulting higher mortality rates in the elderly is required before an increased incidence of infection in the elderly can be attributed to an increase in their susceptibility to infection because of defective immune or host defense mechanisms.