Infectious Diseases Definition

Infectious diseases are illnesses that result from INFECTION with microorganisms, also called microbes. Doctors who treat people who have infectious diseases are internists (who treat adults) or pediatricians (who treat children) who subspecialize in infectious diseases. Other sections in The Facts On File Encyclopedia of Health and Medicine discuss infections specific to individual body systems.

An infection occurs when microbes—bacteria, fungi, parasites, viruses—and other pathogens (infectious agents) such as prions invade the body. The infection causes illness (becomes a disease) when it alters in some deleterious fashion the functions of the body. Some infectious diseases are primarily a health concern only to the people who have them, such as NECROTIZING FASCIITIS, TOXIC SHOCK SYNDROME, and CANDIDIASIS. These illnesses are noncommunicable; they do not spread to other people.

How Infectious Diseases Spread

In some situations infections affect people who have no contact with one another but somehow share a generalized source of contamination. These infections, such as occur with WATERBORNE ILLNESSES in which drinking water or recreational water contains pathogens that people consume, or in LEGIONNAIRES’ DISEASE, in which building heating and air-conditioning systems disperse Legionella pneumophilia bacteria to all who breathe the building’s air, are communicable. Though contact among infected individuals may spread the infection, the typical mode of transmission is contact with the common source.

Numerous infections spread from one person to another, directly such as through touching or sharing bodily fluids or indirectly through sneezing or coughing. These illnesses are not only communicable but also contagious: they spread easily, rapidly, and often extensively. MEASLES, for example, is one of the most highly contagious communicable diseases; 90 percent of people exposed to the virus become ill with the disease. COLDS, infectious mononucleosis (EPSTEIN-BARR VIRUS infection), and INFLUENZA are among the most common contagious diseases in the United States.

Epidemics occur when large numbers of people become ill with a communicable or contagious disease. Throughout history these waves of infection decimated families, cities, countries, and even entire civilizations. Smallpox, measles, bubonic plaque, gonorrhea, syphilis, and influenza are among the infections that raged through populations. An infectious disease is endemic when it is always present at relatively the same rate of infection within a certain geographic region, environment, or population of people. Malaria is endemic in Africa, for example, and consistently sickens thousands of people.

Globalization of Infections

Until the 1950s geographic boundaries confined most infectious diseases, not because pathogens (disease-causing microbes) had much regard for natural or national borders but because few people traveled very far from home. The advent of commercial air flight changed all that. By the 1970s air travel could whisk a person literally halfway around the world in less time than it took to drive from San Francisco to Seattle. Few infectious diseases today remain localized, though the risk of infection with them varies widely. HIV/AIDS, SEVERE ACUTE RESPIRATORY SYNDROME (SARS), and INFLUENZA stand as stark evidence that microbes, too, travel the world.

List of Contagious Infectious Diseases


Infectious Diseases in Medical History

Infectious diseases have mystified and plagued humanity for ages. Tuberculosis, smallpox, cholera, typhoid FEVER, and the plague itself (“Black Death”) were for centuries the leading causes of disability, disfigurement, and death. Mummified remains from ancient Egypt show evidence of smallpox and tuberculosis. Disfigurement resulting from smallpox was so common through the 18th century that artists routinely painted portraits that discreetly masked or simply did not portray the extensive scars the disease left on the faces of those who survived the illness.

Hippocrates wrote of “phthisis”—Greek for consumption, an apt name for tuberculosis, the disease that slowly wasted away the lives of those infected. Manuscript fragments recovered from 7th century China reference measles. Ancient Greek documents record outbreaks of “pestilence” that were likely epidemics of measles, smallpox, and perhaps plague.

For centuries doctors believed infectious diseases like tuberculosis represented some sort of inborn weakness in a family because family members often had the same illness, generation after generation. Of course, doctors today know the true reason such illnesses affected entire families: infectious diseases like tuberculosis spread from person to person, and living in close contact makes it easier if not inevitable for them to spread.

The Birth of Vaccination and The Death of a Scourge

In the summer of 1796, eight-year-old James Phipps became the first success story in an effort that would reach fruition nearly 200 years later. Country doctor Edward Jenner (1749–1823) made two scratches on the boy’s arm with a lancet dipped in the fluid from a smallpox sore. Nothing happened. Not then, not 14 days later when the characteristic sores of smallpox should have started erupting. The scratches healed and James remained healthy.

Six weeks earlier, Jenner had performed a similar procedure using the fluid from a cowpox sore, a much milder form of illness that doctors today know develops from infection with a virus closely related to the virus that causes smallpox. Edward Jenner did not know this but had observed that milkmaids and farm hands who recovered from cowpox did not get smallpox. Young James became ill with cowpox, as Jenner expected he would, and then soon recovered—also as Jenner expected he would. Ironically, as an adult James Phipp nearly lost his life to another infectious disease endemic throughout history, tuberculosis.

In 1966 the World Health Assembly formalized a global smallpox eradication program with vaccination, Jenner’s discovery, as its foundation. The first year of the program, 15 million people throughout the world contracted smallpox; nearly a third of them died. Ten years later, on October 26, 1977, Somalian Ali Mao Moallin became the last person in the world to acquire naturally occurring smallpox (he survived). In 1980, the World Health Organization officially declared smallpox eradicated worldwide and advised countries to end vaccination programs.

Vaccination Today

Today vaccination is the cornerstone of infectious disease control and preventive medicine. Vaccines confer IMMUNIZATION for numerous infectious diseases. Many countries, including the United States, routinely administer set schedules of vaccines to children, giving them lifelong immunity that protects them from infection with diseases such as POLIOMYELITIS, MUMPS, MEASLES, CHICKENPOX, RUBELLA, PERTUSSIS (whooping COUGH), and Haemophilus influenzae type b (Hib).

Microbes and The Mechanisms of Infection

As early as the 16th century some scientists postulated the existence of unseen organisms as accountable for disease. The development of the microscope gave proof to the existence of such organisms; proving their connection to disease was more difficult. The first to succeed did so in a major way. German physician Robert Koch (1843–1910) isolated and cultivated Mycobacterium tuberculosis, the MICROBE responsible for the Western world’s most pervasive and devastating disease. By the 19th century, tuberculosis infected so many people that it was more common than not. What puzzled doctors was why some people became ill and others did not.

Koch solved this mystery in 1882 when he demonstrated the ability of M. tuberculosis to cause tuberculosis infection. The methods of vaccination successful in preventing smallpox, anthrax, and other infectious diseases did not work with tuberculosis, however. Not until biochemist Selman Waksman (1888–1973) discovered streptomycin, a powerful antibiotic, in 1943 were doctors finally able to gain the upper hand against tuberculosis. Waksman received the Nobel Prize in Physiology or Medicine in 1952 for his work.

Through their work to understand a disease prevalent among livestock in the 19th century, foot-and-mouth disease, German researchers Friedrich Loeffler (1852–1915) and Paul Frosch (1860–1928) expanded the spectrum of pathogens. The pair postulated the existence of a particle smaller than bacteria caused the infectious disease. However, they lacked the technology to visualize such a particle. The development of the electron microscope in 1939 gave scientists the ability to see these smallest of infective agents, viruses.

Breakthrough Research and Treatment Advances

Molecular medicine advances in the late 20th century gave another enormous boost to the fight against infectious diseases. In 1995 the bacterium Haemophilus influenzae, an insidious microbe responsible for numerous pulmonary and gastrointestinal diseases, became the first pathogen for which researchers unraveled the genetic code. The advance led to improvements in vaccines and treatments for H. influenzae infections as well as other bacterial diseases.

Molecular medicine also has provided tremendous breakthroughs in understanding the modus operandi of viruses such as HIV (human immunodeficiency virus), a Machiavellian retrovirus that subverts the immune system itself to perpetuate its own survival. These breakthroughs have paved the way for new antiviral medications that target specific molecular mechanisms of HIV, slowing its progress, and show promise for the development of a vaccine that can prevent HIV infection and AIDS.

As researchers gain insight into the adaptive mechanisms of pathogens such as bacteria and viruses, they are able to develop new drugs—and drugs that work in new ways—to treat the infections these pathogens cause. This is particularly important in light of the alarming rise in DRUGresistant infections in diseases such as tuberculosis, GONORRHEA, and staphylococcal pneumonia. New viruses also threaten public health, placing renewed emphasis on vaccines and infection control measures to stop their spread. Though the control and eradication of many infectious diseases represent many of medicine’s greatest triumphs, many of medicine’s greatest challenges remain these same factors.

Information on Infectious Diseases
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