Infections are more likely and usually more severe in older people than in younger people for several reasons:

• Aging reduces the immune system’s effectiveness ( see Effects of Aging on the Immune System).

• Many long-term (chronic) disorders that are common among older people—such as chronic obstructive pulmonary disease, cancer, and diabetes mellitus—also increase the risk of infection.

• Older people are more likely to be in a hospital or a nursing home, where the risk of acquiring a serious infection is greater. In hospitals, the widespread use of antibiotics allows antibiotic-resistant microorganisms to thrive, and infections with these microorganisms are often more difficult to treat than infections acquired at home or in the community ( see Hospital-Acquired Infections).