INFECTION

in medicine, the invasion of the body by any of various infectious agentsincluding bacteria, viruses, fungi, protozoans, and wormsand the reaction of the body to them or to their toxins. When health is not altered, the process is termed a subclinical infection. When health is altered, the process is called an infectious disease. There are various routes that infectious agents can take into the human body. Conditions such as influenza and the common cold occur when their respective viruses attack the mucous membranes that line the upper respiratory tract. Pneumonias, diseases of the lower respiratory tract, can be caused by almost any infectious agent. If bacteria, viruses, or other agents get into the bloodstream, they can attack the cardiovascular and central nervous systems, producing such diseases as rheumatic fever or meningitis. Gastrointestinal diseases, ranging from salmonella food poisoning to life-threatening typhoid fever and cholera, generally result from the ingestion of contaminated food or water. Sexually transmitted diseases are acquired during intercourse or other sexual activity. Bacteria can invade the bones and joints, causing osteomyelitis or septic arthritis, or they can invade the skin, causing impetigo or leprosy. Other diseases, such as tetanus, occur when a skin wound is contaminatedin this case, with soil carrying tetanus bacilli. A few diseases can arise via more than one route. Different forms of anthrax, for example, are produced when anthrax bacilli are breathed in, acquired through skin abrasions, or ingested. Insects are responsible for injecting some infectious agents through the skin into the human bloodstream. The plasmodia that cause malaria and the viruses that cause yellow fever are transmitted by mosquitoes. Typhoid fever and several other diseases can be spread by human carriers who harbour the microbes long after they have recovered from infection. Animals also act as reservoirs for microbes. Rabies virus can be contracted from the bite of a rabid animal, and leptospirosis is caused by bacteria that pass between human beings and other mammals. Following local infection or entrance of a microorganism into the bloodstream, white blood cells attack the invader, and antibodies or antitoxins are produced as needed. The resulting active immunity can be lifelong, as that against measles, or short-lived. This process provides the theoretical basis for long-lasting active immunization by vaccination with a variety of antigens. Passive immunity, the introduction of antibodies or antitoxins into the body, is temporary because the body eventually eliminates them as foreign substances. Body temperature often rises in response to infection. Although newly recognized microbes emerge occasionally (such as Legionella pneumophila, the cause of Legionnaire's disease, in 1976 or the HIV virus, the cause of AIDS, in the early 1980s), and although there are serious difficulties in arresting the spread of some conditions (particularly sexually transmitted ones), some diseases have largely been conquered. Smallpox became virtually extinct in the late 1970s, and diseases such as measles, diphtheria, and poliomyelitis are rare in regions in which modern medical techniques are available. Still, the burden of illness and death attributable to infectious organisms remains massive, particularly in regions of poor sanitation, poor nutrition, and crowding. in medicine, the invasion of the body by any of various infectious agentsincluding bacteria, viruses, fungi, protozoans, and wormsand the reaction of the body to them or their toxins. When health is not altered, the process is termed a subclinical infection. When health is altered, the process is called an infectious disease. Thus, a person may be infected but not have an infectious disease. This principle is illustrated by the use of vaccines for the prevention of infectious diseases. For example, a virus such as that which causes measles may be partially inactivated (attenuated) and used as an immunizing agent. The immunization is designed to produce a measles infection in the recipient but generally causes no discernible alteration in the state of health. It produces immunity to measles without producing a clinical illness (an infectious disease). Among the agents of infectious disease are microorganisms, the microscopic or submicroscopic germs such as viruses, bacteria, and protozoans. Parasites such as the acarids, or itch mites, of scabies are barely visible, and helminths, or worms, may grow to be several feet long. The only characteristic that all of these have in common is their dependence, for all or part of their life cycles, on their ability to parasitize, or live on, human beings. To survive and multiply, a virus must get inside a living cell, but many bacteria can live and multiply for a time on nonliving matter, while helminths, for part of their lives, lead an independent existence. One function of an organism that does not correlate with size is the ability to cause serious disease. The virus of poliomyelitis, for example, is only about 25 millimicrometres (a micrometre, or micron, is 0.001 millimetre [0.00004 inch] and a millimicrometre is 0.001 micrometre) in diameter. The most important barriers to invasion of the human host by microorganisms are the skin and mucous membranes (the tissues that line the nose, mouth, and upper respiratory tract). When these tissues have been broken or affected by earlier disease, invasion by microorganisms may occur. These microorganisms may produce a local infectious disease, such as boils, or may invade the bloodstream and be carried throughout the body, producing generalized bloodstream infection (septicemia) or localized infection at a distant site, such as meningitis (an infection of the coverings of the brain and spinal cord). Infectious agents can be swallowed in food and drink to attack the wall of the intestinal tract and cause local or general disease. The conjunctiva, which covers the front of the eye, may be penetrated by viruses that cause a local inflammation of the eye or pass into the bloodstream and cause a severe general disease, such as measles or smallpox. Microorganisms can enter the body through the genital tract, setting up the acute inflammatory reaction of gonorrhea in the genital and pelvic organs or spreading out to attack almost any organ of the body with the more chronic and destructive lesions of syphilis. Even before birth, viruses and other infectious agents can pass through the placenta and attack developing cells, so that an infant may be diseased or deformed at birth. From conception to death, then, human beings are targets for attack by multitudes of other living organisms, all of them competing for a place in the common environment. The air people breathe, the soil they walk on, the waters and vegetation around them, the buildings they inhabit and work in, all can be populated with forms of life that are dangerous. Domestic animals may harbour organisms that are a threat, and wildlife teems with agents of infection that can afflict human beings with serious disease. The human body is not without defenses against these threats, for it is equipped with sensitive mechanisms that react quickly and specifically against disease organisms when they attack, and survival throughout the ages has depended largely on these reactions. The human environment, in the biological sense, is mainly a hostile one. Human beings have learned to partially control it, but they cannot subdue it, and sometimes a minor change in the environment may lead to unforeseen alterations in the balance between people and their biological competitors. Andrew Barnett Christie Ralph D. Feigin Renu Garg Additional reading Gerald L. Mandel, R. Gordon Douglas, Jr., and John E. Bennett (eds.), Principles and Practice of Infectious Diseases, 3rd ed. (1990), is a modern textbook that the general reader may find informative and useful. Richard E. Behrman and Robert M. Kliegman (eds.), Nelson Textbook of Pediatrics, 14th ed. (1992), deals with the infectious diseases of children and is well written and illustrated. Jack S. Remington and Jerome O. Klein (eds.), Infectious Diseases of the Fetus and Newborn Infant, 3rd ed. (1990), an advanced textbook, includes sections that discuss some aspects of pediatric infections at a nontechnical level. R.T.D. Emond and H.A.K. Rowland, A Color Atlas of Infectious Diseases, 2nd ed. (1987), reveals by the use of colour photographs the actual clinical features associated with various infectious diseases. Andre Siegfried, Routes of Contagion (also published as Germs and Ideas: Routes of Epidemics and Ideologies, 1965; originally published in French, 1960); and Henry E. Sigerist, Civilization and Disease (1943, reissued 1970), deal with the effect of disease on human life and history; both are nontechnical. Virginia Berridge and Philip Strong (eds.), AIDS and Contemporary History (1993), also for the general reader, similarly takes a historical perspective but with regard to a contemporary epidemic. Frank M. Burnet, The Background of Infectious Diseases in Man (1946), outlines the problems posed by infectious diseases in the middle of the 20th century; the fact that many of the problems have since been solved adds to the interest of the book. Clifford Horton-Smith (ed.), Biological Aspects of the Transmission of Disease (1957), deals with the mechanics and interrelationships of infection in plants, animals, and humans; the level of writing varies from the very technical to the semipopular. King K. Holmes et al., Sexually Transmitted Diseases, 2nd ed. (1990), is a well-written text on a facet of infectious disease that is assuming increasing importance in society. William H. Mcneill, Plagues and Peoples (1976, reprinted 1989), a book for the general reader, describes the dramatic impact of infectious diseases on the rise and fall of civilizations. John F.D. Shrewsbury, A History of Bubonic Plague in the British Isles (1970), gives a scholarly but highly readable description of the plague. Kenneth Mellanby, Pesticides and Pollution, 2nd rev. ed. (1970), reviews the problems presented by pollution in the modern world, many of which derive from attempts to control the spread of disease in humans and nature. James M. Alston, A New Look at Infectious Disease (1967), shows briefly how, as one problem in the field of infectious disease is solved, another takes its place. J.A. Boycott, Natural History of Infectious Disease (1971), discusses infectious disease as just one aspect of parasitisma study, in simple terms, of host-parasite relationships. Paul W. Ewald, Evolution of Infectious Disease (1993), an accessible work, is the first to present a Darwinian perspective on infectious disease. Paul Chester Beaver and Rodney Clifton Jung, Animal Agents and Vectors of Human Disease, 5th ed. (1985), a basic textbook for the medical student, surveys the identification and life cycles of animal parasites in humans and the epidemiology and treatment of the associated diseases. John J. Marchalonis (ed.), Immunobiology of Parasites and Parasitic Infections (1984), written for immunologists but accessible to other technical readers, is an overview covering diverse topics in the immunobiology and immunopathology of protozoan and metazoan parasites. Kenneth S. Warren (ed.), Immunology and Molecular Biology of Parasitic Infections, 3rd ed. (1993), an advanced textbook, explores the mechanistic aspects of parasitic diseases at the cellular and molecular levels. A.B. Christie, Infectious Diseases: With Chapters on Venereal Diseases, 5th ed. (1968), gives a simple account of the clinical and social aspects of infectious diseases. Two works for the general reader focusing on the role of human behaviour in the exposure to and transmission of disease are Neil A. Croll and John H. Cross (eds.), Human Ecology and Infectious Diseases (1983), on infectious diseases in general; and Marie A. Muir, The Environmental Contexts of AIDS (1991), on AIDS in particular. Christie's Infectious Diseases: Epidemiology and Clinical Practice, 4th ed., 2 vol. (1987), is an advanced textbook, but with much about the nature of infection that the nontechnical reader will find useful. A.B. Christie and Mary C. Christie, Food Hygiene and Food Hazards for All Who Handle Food, 2nd ed. (1977), discusses nontechnically the problems of food-borne infections. Frank L. Horsfall, Jr., and Igor Tamm (eds.), Viral and Rickettsial Infections of Man, 4th ed. (1965); and Alfred S. Evans (ed.), Viral Infections of Humans: Epidemiology and Control, 3rd ed., completely rev. and expanded (1991), are advanced postgraduate textbooks but contain material, especially on epidemiology, to which the general reader might refer. Stanley J. Cryz, Jr. (ed.), Vaccines and Immunotherapy (1991), a graduate-level textbook, provides both a thorough introduction to immunotherapy and a comprehensive review of the development of vaccines. Ronald W. Ellis (ed.), Vaccines: New Approaches to Immunological Problems (1992), concentrates on the technical approaches used to design and develop new vaccines. Andrew Barnett Christie Ralph D. Feigin Renu Garg The Editors of the Encyclopdia Britannica Effects of environment on human disease Human activity Social patterns Human beings are social animals. As a result, human social habits and circumstances influence the spread of infectious agents. Poorer families tend to live in more crowded conditions, which facilitate the passage of disease-causing organisms from one person to another. This is true whether the germs pass through the air from one respiratory tract to another, or whether they are bowel germs and depend for their passage on close personal hand-to-mouth contact or on lapses of sanitation and hygiene. The composition of the family unit is also important. In families with infants and preschool children, infection spreads more readily, for these youngsters are both more susceptible to infection and, because of their faulty hygiene habits, more likely to share their microbes with other family members and with friends. Because of this close and confined contact, infectious agents are spread more rapidly. Distinction must be made between disease and infection. The virus of poliomyelitis, for example, spreads easily in conditions of close contact (infection), but it usually causes no active disease. When it does cause active disease, it attacks older people much more severely than the young. Children in more crowded homes, for example, are likely to be infected at an early age and, if illness results, it is usually mild. In less crowded conditions, young children are exposed less often to infection; when they first encounter the virus at an older age, they tend to suffer more severely. The difference between infection and disease is seen even more clearly in the outcome of the infection. Under conditions of poor hygiene, poliovirus spreads rapidly in early childhood, leading more often to immunity than to illness; under high standards of hygiene, young children are exposed less frequently and fewer develop immunity in early life, with the result that paralytic illness, a rarity under the former conditions, is seen frequently in older children and adults. The pattern of infection and disease, however, can be changed. In the case of the poliomyelitis virus, only artificial immunization can abolish both infection and disease. Population density Density of population does not of itself determine the ease with which infection spreads through a population. In New York City, with its many high-rise dwellings, the density of the population per square mile is much greater than in some of the world's older cities, but the hepatitis virus, for example, spreads much faster in the latter. A family in a New York City apartment may never see the inhabitants of most of the other apartments in the block, while neighbours in an ancient Asian city are in daily contact. In New York state, the incidence of infectious hepatitis has been shown to vary inversely with population densityto be lowest in the city itself, higher in other urban areas of the state, and highest in rural areas. This pattern, of course, reflects the population's chances of contact and social habits, as well as its density.