The appearance of many well-recognized "diseases of aging" tends to mask a similar rise in the susceptibility of the aged to infections. The immune response, particularly cell-mediated immunity, declines in efficiency with age, but this change alone does not explain the increased occurrence of infections. Those infections that show the greatest increase in aged people are infections of certain organ systems, e.g., the lung, for which the increase in susceptibility can be explained largely by the changes in structure and function that occur with age. An infection during old age may also be induced by another condition such as degenerative disease, catheterization, malnutrition, or another infection. Thus there appears to be a network of factors in the aging body, including degenerative changes and the declining immune response, that interact with and compound each other to markedly increase susceptibility to infection.