Aging reduces the immune system’s effectiveness (see Effects of Aging on the Immune System Effects of Aging on the Immune System The immune system is the body's defense against foreign or dangerous invaders. Such invaders include Microorganisms (commonly called germs, such as bacteria, viruses, and fungi) Parasites (such... read more ).

Many long-term (chronic) disorders that are common among older people—such as chronic obstructive pulmonary disease Chronic Obstructive Pulmonary Disease (COPD) Chronic obstructive pulmonary disease is persistent narrowing (blocking, or obstruction) of the airways occurring with emphysema, chronic obstructive bronchitis, or both disorders. Cigarette... read more , cancer, and diabetes mellitus Diabetes Mellitus (DM) Diabetes mellitus is a disorder in which the body does not produce enough or respond normally to insulin, causing blood sugar (glucose) levels to be abnormally high. Urination and thirst are... read more —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 Hospital-Acquired Infections People who are admitted to the hospital are at risk of acquiring an infection there. Such infections are called nosocomial infections. In the United States, about 4-5% of people who are hospitalized... read more ).