Epidemiology and Unique Aspects of Aging and Infectious Diseases Thomas T. Yoshikawa From the Department of Internal Medicine, Charles R. Drew University of Medicine and Science and Martin Luther King, Jr.–Charles R. Drew Medical Center, Los Angeles, California The elderly population will grow rapidly over the next 25 years. The majority of patients with serious or life-threatening infections will be old. It is imperative that primary care physicians and infectious diseases specialists become aware of and knowledgeable about the special and unique aspects of infections in the geriatric population. The Epidemiology of Infection and Aging Up until the mid-20th century, infectious diseases were the major causes of death and disability in the United States. Worldwide, major outbreaks of smallpox, diphtheria, cholera, typhoid fever, typhus, plague, and tuberculosis accounted for the deaths of millions of people, and such diseases as scarlet fever, rheumatic fever, mumps, measles, pertussis, syphilis, and poliomyelitis caused not only death but also severe disabilities, deformities, functional incapacity, and social rejection [1]. The discovery of the germ theory of disease brought medical advances such as sanitation, public health measures, antisepsis, antibiotics and vaccination, which contributed substantially to the reduction of deaths and morbidity of infectious diseases in industrialized countries in the second half of the 20th century. Ironically, today infectious diseases are still responsible for onethird of all deaths in the world; the World Health Organization estimates that nearly 50,000 people die each day throughout the world directly from an infectious disease [2]. In 1900, when infections were the leading cause of death, the average life expectancy at birth in the United States was ∼47 years (46 years for males and 48 years for females) [3]; only 4% of the entire population in the United States was aged >65 years [4]. Childhood mortality was extremely high. However, as the aforementioned advances in medicine occurred in the early 20th century, there was a concomitant reduction in deaths of children (as well as adults) due to infectious diseases. Consequently, the average life expectancy at birth in the United States rapidly increased over the latter half of this past century. Presently, the average life expectancy at birth in this country is ∼75 years (73 years for males and 80 years for females) with slight variations depending on ethnic or racial background [3]. The elderly population (aged >65 years) presently account for 13% of the total United States population [4]. It is anticipated Received 14 January 2000; electronically published 30 June 2000. Reprints and correspondence: Dr. Thomas T. Yoshikawa, Dept. of Internal Medicine, King-Drew Medical Center, 12021 S. Wilmington Ave., Los Angeles, CA 90059 (toyoshik@cdrewu.edu). Clinical Infectious Diseases 200030:931–33 q 2000 by the Infectious Diseases Society of America. All rights reserved. 1058-4838/2000/3006-0015$03.00 that the current population of ∼34 million persons aged >65 years will nearly double over the next 30 years, to 21% of all Americans. Equally important, the percentage of elderly persons who are very old (aged >80 years) will grow the most rapidly over the next quarter century. Clinically, this subgroup of elderly is relevant because they are the most vulnerable to serious infectious diseases and are at greatest risk for death and complications from infections. As stated earlier, the major causes of death up through the early 20th century were related to infectious diseases. Now the top 3 causes of death for the general population are heart diseases, cancers, and stroke; this also holds true for the elderly population. However, among aging adults, pneumonia and influenza are the fourth most common cause of death, diabetes mellitus and its complications including infections are the sixth, and complications from bacteremia is the ninth