Humans may hold dominance over most other life-forms on Earth, but a few varieties of organism have long held mastery over us. Ironically, these life-forms, including bacteria and viruses, are so small that they cannot be seen, and this, in fact, has contributed to their disproportionate influence in human history. For thousands of years, people attributed infection to spiritual causes or, at the very least, to imbalances of "humors," or fluids, in the human body. Today germ theory and antisepsis—the ideas that microbes cause infection and that a clean body and environment can prevent infections—are ingrained so deeply that we almost take them for granted. Yet these concepts are very recent in origin, and for a much longer span of human history people quite literally wallowed in filth—with predictable consequences.
The term infection refers to a state in which parasitic organisms attach themselves to the body, or to the inside of the body, of another organism, causing contamination and disease in the host organism. Parasite refers generally to any organism that lives at the expense of another organism, on which it depends for support. Numerous parasites and the diseases they cause are discussed in the essay Parasites and Parasitology; in the present context, we are concerned primarily with infections that relate to bacteria and viruses.
Almost all infections contracted by humans are passed along by other humans or animals.
Infections fall into two general categories: exogenous, or those that originate outside the body, and endogenous, which occur when the body's resistance is lowered. Examples of exogenous infection include catching a cold by drinking after someone else from the same glass; coming down with salmonella after ingesting under-cooked eggs, meat, or poultry; getting rabies from a dog bite; or contracting syphilis, AIDS (acquired immunodeficiency syndrome), or some other sexually transmitted disease from an infected partner.
Any number of factors—lack of sleep, prolonged exposure to extreme cold or moisture, and so on—can lower the body's resistance, opening the way for an endogenous infection. Malnutrition, illness, and trauma also can be factors in endogenous infection. Substance abuse, whether it be the use of tobacco in its many forms, excessive drinking, or drug use, lowers the body's resistance. Furthermore, all of these behaviors tend to be coupled with poor eating habits, which invite infection by denying the body the nutrients it needs.
A whole array of terminology attends the study of infection and infectious diseases, a subject that is touched upon in the present context but explored at length in its own essay as well. Among these terms are the names for the different branches of study relating to infection, its agents, and the resulting diseases. Although germ theory is a term (defined later) that is used widely in the context of infection, germ itself—a common word in everyday life—is not used as much as microorganism or pathogen. The latter word refers to disease-carrying parasites, which are usually microorganisms. Two of the principal types of pathogen, bacteria and viruses, are discussed later in this essay.
Words relating to the effects of infectious agents include epidemic, an adjective meaning "affecting or potentially affecting a large proportion of a population"; as a noun, the word refers to an epidemic disease. Pandemic also doubles as an adjective, meaning "affecting an extremely high proportion of a population over a wide geographic area," and a noun, referring to a disease of pandemic proportions. Areas of study relating to pathogens, their effects, and the prevention of those effects include the following.
In addition, there are several terms relating to the prevention of infection.
Some of these words appear in this essay and others in related essays on infectious diseases and immunity.
Five major groups of microorganisms are responsible for the majority of infections. They include protozoa and helminths, or worms—both of which are considered in Parasites and Parasitology—as well as bacteria and viruses. Bacteria and viruses often are discussed, along with fungi (the fifth major group), in the context of infection and infectious diseases. In the present context, however, we limit our inquiry to viruses and bacteria.
Bacteria are very small organisms, typically consisting of one cell. They are prokaryotes, a term referring to a type of cell that has no nucleus. In eukaryotic cells, such as those of plants and animals, the nucleus controls the cell's functions and contains its genes. Genes carry deoxyribonucleic acid (DNA), which determines the characteristics that are passed on from one generation to the next. The genetic material of bacteria is contained instead within a single, circular chain of DNA.
Members of kingdom Monera, which also includes blue-green algae (see Taxonomy), bacteria generally are classified into three groups based on their shape: spherical (coccus), rodlike (bacillus), or spiralor corkscrew-shaped (spirochete). Some bacteria also have a shape like that of a comma and are known as vibrio. Spirochetes, which are linked to such diseases as syphilis, sometimes are considered a separate type of creature; hence, Monera occasionally is defined as consisting of blue-green algae, bacteria, and spirochetes.
The cytoplasm (material in the cell interior) of all bacteria is enclosed within a cell membrane that itself is surrounded by a rigid cell wall. Bacteria produce a thick, jellylike material on the surface of the cell wall, and when that material forms a distinct outer layer, it is known as a capsule. Many rod, spiral, and comma-shaped bacteria have whiplike limbs, known as flagella, attached to the outside of their cells. They use these flagella for movement by waving them back and forth. Other bacteria move simply by wiggling the whole cell back and forth, whereas still others are unable to move at all.
Bacteria most commonly reproduce by fission, the process by which a single cell divides to produce two new cells. The process of fission may take anywhere from 15 minutes to 16 hours, depending on the type of bacterium. Several factors influence the rate at which bacterial growth occurs, the most important being moisture, temperature, and pH, or the relative acidity or alkalinity of the substance in which they are placed.
Bacterial preferences in all of these areas vary: for example, there are bacteria that live in hydrothermal vents, or cracks in the ocean floor, where the temperature is about 660°F (350°C), and some species survive at a pH more severe than that of battery acid. Most bacteria, however, favor temperatures close to that of the human body—98.6°F (37°C)—and pH levels only slightly more or less acidic than water. Since they are composed primarily of water, they thrive in a moist environment.
One of the interesting things about bacteria is their simplicity, coupled with the extraordinary complexity of their interactions with other organisms. As simple as bacteria are, however, viruses are vastly more simple. Furthermore, the diseases they can cause in other organisms are at least as complex as those of bacteria, and usually much more difficult to defeat. Whereas there are "good" bacteria, as we shall see, scientists have yet to discover a virus whose impact on the world of living things is beneficial. There is something downright creepy about viruses, which are not exactly classifiable as living things; in fact, a virus is really nothing more than a core of either DNA or RNA (ribonucleic acid), surrounded by a shell of protein.
Two facts separate viruses from the world of the truly living. First, unlike all living things (even bacteria), viruses are not composed of even a single cell, and, second, a virus has no life if it cannot infect a host cell. When we say "no life" in this context, we truly mean no life. Although parasites, including bacteria and those species discussed in Parasites and Parasitology, depend on other organisms to serve as hosts, they can live when they are between hosts. They are rather like a person between jobs: without other means of support, the person eventually will go broke or starve, but typically such a person can hang on for a few months until he or she finds a new job. A virus without a host, on the other hand, is simply not alive—not dead, like a formerly living thing, but more like a machine that has been switched off.
Once a virus enters the body of a host, it switches on, and the result is truly terrifying. In order to produce new copies of itself, a virus must use the host cell's reproductive "machinery"—that is, the DNA. The newly made viruses then leave the host cell, sometimes killing it in the process, and proceed to infect other cells within the organism. As for the organisms that viruses target, their potential victims include the whole world of living things: plants, animals, and bacteria. Viruses that affect bacteria are called bacteriophages, or simply phages. Phages are of special importance, because they have been studied much more thoroughly than most viruses; in fact, much of what virologists now know about viruses is based on the study of phages.
Not all bacteria are harmful; in fact, some even are involved in the production of foods consumed by humans. For example, bacteria that cause milk to become sour are used in making cottage cheese, buttermilk, and yogurt. Vinegar and sauerkraut also are produced by the action of bacteria on ethyl alcohol and cabbage, respectively. Other bacteria, most notably Escherichia coli (E. coli) in the human intestines, make it possible for animals to digest foods and even form vitamins in the course of their work. (See Digestion for more on these subjects.) Others function as decomposers (see Food Webs), aiding in the chemical breakdown of organic materials, while still others help keep the world a cleaner place by consuming waste materials, such as feces.
Despite its helpful role in the body, certain strains of E. coli are dangerous pathogens that can cause diarrhea, bloody stools, and severe abdominal cramping and pain. The affliction is rarely fatal, though in late 1992 and 1993 four people died during the course of an E. coli outbreak in Washington, Idaho, California, and Nevada. More often the outcome is severe illness that may bring on other conditions; for example, two teenagers among a group of 11 who became sick while attending a Texas cheerleading camp had to receive emergency appendectomies. The pathogen is usually transmitted through under-cooked foods, and sometimes through other means; for example, a small outbreak in the Atlanta area in the late 1990s occurred in a recreational water park.
Many bacteria attack the skin, eyes, ears, and various systems in the body, including the nervous, cardiovascular, respiratory, digestive, and genitourinary (i.e., reproductive and urinary) systems. The skin is the body's first line of defense against infection by bacteria and other microorganisms, although it supports enormous numbers of bacteria itself. Bacteria play a major role in a skin condition that is the bane of many a young man's (and, less frequently, a young woman's) existence: acne. Pimples or "zits," known scientifically as Acne vulgaris, constitute one of about 50 varieties of acne, or skin inflammation, which are caused by a combination of heredity, hormones, and bacteria—particularly a species known as Propionibacterium acnes. When a hair follicle becomes plugged by sebum, a fatty substance secreted by the sebaceous, or oil, glands, this forms what we know as a blackhead; a pimple, on the other hand, results when a bacterial infection, brought about by P. acnes, inflames the blackhead and turns it red. For this reason, antibiotics may sometimes cure acne or at least alleviate the worst symptoms.
Acne may seem like a life-and-death issue to a teenager, but it goes away eventually. On the other hand, toxic shock syndrome (TSS), caused by other bacteria at the surface of the skin—species of Staphylococcus and Streptococcus—can be extremely dangerous. The early stages of TSS are characterized by flulike symptoms, such as sudden fever, fatigue, diarrhea, and dizziness, but in a matter of a few hours or days the blood pressure drops dangerously, and a sunburn-like rash forms on the body. Circulatory problems arise as a result of low blood pressure, and some extremities, such as the fingers and toes, are deprived of blood as the body tries to shunt blood to vital organs. If the syndrome is severe enough, gangrene may develop in the fingers and toes.
In 1980, several women in the United States died from TSS, and several others were diagnosed with the condition. As researchers discovered, all of them had been menstruating and using high-absorbency tampons. It appears that such tampons provide an environment in which TSS-causing bacteria can grow, and this led to recommendations that women use lower-absorbency tampons if possible, and change them every two to four hours. Since these guidelines were instituted, the incidence of toxic shock has dropped significantly, to between 1 and 17 cases per 100,000 menstruating women.
Many bacteria produce toxins, poisonous substances that have effects in specific areas of the body. An example is Clostridium tetani, responsible for the disease known as tetanus, in which one's muscles become paralyzed. A related bacterium, C. botulinum, releases a toxin that causes the most severe form of food poisoning, botulism. Salmonella poisoning comes from another genus, Salmonella, which includes S. typhi, the cause of typhoid fever.
With viruses, as we have noted, there is no need even to discuss "good" kinds, because there is no such thing—all viruses are harmful, and most are killers. The particular strains of virus that attack animals have introduced the world to a variety of ailments, ranging from the common cold to AIDS and some types of cancer. Other diseases related to viral infections are hepatitis, chicken pox, smallpox, polio, measles, and rabies.
One reason why physicians and scientists have never found a cure for the common cold is that it can be caused by any one of about 200 viruses, including rhinoviruses, adenoviruses, influenza viruses, parainfluenza viruses, syncytial viruses, echoviruses, and coxsackie viruses. Each has its own characteristics, its favored method of transmission, and its own developmental period. These viruses can be transmitted from one person to another by sneezing on the person, shaking hands, or handling an object previously touched by the infected person. Surprisingly, some more direct forms of contact with an infected person, as in kissing, seldom spread viruses.
A group of viruses called the orthomyxoviruses transmit influenza, an illness usually characterized by fever, muscle aches, fatigue, and upper respiratory obstruction and inflammation. The most common complication of influenza is pneumonia, a disease of the lungs that may be viral or bacterial. The viral form of pneumonia that goes hand in hand with influenza can be very severe, with a high mortality (death) rate; by contrast, bacterial pneumonia, which typically appears five to ten days after the onset of flu, can be treated with antibiotics.
Viruses are tricky. Because their generations are very short and their structures extremely simple, they are constantly mutating (altering their DNA and hence their heritable traits) and thus becoming less susceptible to vaccines. This is the reason why flu vaccine has to be prepared a new each year to target the current strains, and even then the vaccine is far less than universally effective. On the other hand, vaccination has a high rate of success for strains of virus that undergo little mutation—for example, the smallpox virus.
One particularly elusive type of virus is known as a retrovirus, which reverses the normal process by which living organisms produce proteins. Ordinarily, DNA in the cell's nucleus carries directions for the production of new protein. Coded messages in the DNA molecules are copied into RNA molecules, which direct the manufacture of new protein. In retroviruses, that process is reversed, with viral RNA used to make new viral DNA, which then is incorporated into host cell DNA, where it is used to direct the manufacture of new viral protein. Among the diseases caused by retroviruses is AIDS, discussed in Infectious Diseases and The Immune System.
Every day of our lives, we are at war with microorganisms, both individually and as a species. It is a war that has lasted for several million years, with billions of lives in the balance, yet it is an invisible war. Up until a few centuries ago, in fact, we had no idea what we were fighting. Before the advent of germ theory, the most scientific theories of disease blamed them either on an imbalance of "humors" (blood, phlegm, yellow bile, and green bile), or on inhaling bad air. These were the most advanced ideas, the ones held by men of learning; most of the populace, by contrast,
Personal hygiene and public health were completely foreign concepts: not only did people bathe infrequently, but they also thought nothing of throwing trash—including rotting food and even human excrement—into the city streets. This image of trash in the streets may call to mind a city of medieval Western Europe, a place and time widely known for its filth, squalor, and ignorance. Yet such an image also describes Athens during the fifth century B.C., when human imagination, wisdom, and appreciation for beauty reached perhaps their highest points in all of history. In the Athens of Socrates, Herodotus, Hippocrates, and Sophocles, the streets were piled with trash and crawling with vermin. In fact, this lack of concern for cleanliness contributed directly to the end of the Greek golden age, sometimes known as the Age of Pericles, after Athens's great leader (495-425 B.C.)—who died in a great plague that swept the germ-ridden city.
The first inkling of any etiology other than that of imbalanced humors and demons was the work of the Italian physician Girolamo Fracastoro (ca. 1483-1553), who put forth the theory that disease is caused by particles so small they are almost imperceptible. The invention of the microscope in 1590 made it possible to glimpse those particles, which Holland's Anton van Leeuwenhoek (1632-1723)—the first human being to observe bacteria and other microorganisms—dubbed animalcules, or "tiny animals." The German scholar Athanasius Kircher (1601-1680) also observed "tiny worms" in the blood and pus of plague victims and theorized that they were the source of the infection. This was the first theory that dealt with microbial agents as infectious organisms.
In 1848 Ignaz P. Semmelweis (1818-1865), a Hungarian physician working in German hospitals, came up with a novel idea: after examining the bodies of women who had died of puerperal (childbed) fever, he suggested that doctors should wash their hands in a solution of chlorinated lime water before touching a pregnant patient. Semmelweis's idea resulted in a drastic reduction of puerperal fever cases, but his colleagues denounced his outlandish notion as a useless and foolish waste of time. Six years later, in 1854, modern epidemiology was born when the English physician John Snow (1813-1858) determined that the source of a cholera epidemic in London could be traced to the contaminated water of the Broad Street pump. After he ordered the pump closed, the epidemic ebbed—and still many physicians refused to believe that invisible organisms could spread disease.
A major turning point came just three years later, in 1857, when the great French chemist and microbiologist Louis Pasteur (1822-1895) discovered that heating beer and wine to a certain temperature killed bacteria that caused these liquids to spoil or turn into vinegar. Thus was born the process of pasteurization, still used today to purify such foods as milk, because, as Pasteur observed, "There are similarities between the diseases of animals or man and the diseases of beer and wine." Pasteur also dealt the final blow to spontaneous generation, a centuries-old belief that living organisms could originate from nonliving matter. As he showed in 1861, microorganisms present in the air can contaminate solutions that seem sterile.
Then, in 1876, the German physician Robert Koch (1843-1910) proved what Kircher had postulated two centuries earlier: that bacteria can cause diseases. Koch showed that the bacterium Bacillus anthracis was the source of anthrax in cattle and sheep and generalized the methodology he had used in that situation to form a specific set of guidelines for determining the cause of infectious diseases. Known as Koch's postulates, these guidelines define a truly infectious agent as one that can be isolated from an infected animal, cultured in a laboratory setting, introduced into a healthy animal to produce the same infection as in the first animal, and isolated again from the second animal. These ideas formed the basis of research into bacterial diseases and are still dominant in the sciences devoted to the study of disease.
Koch's postulates helped usher in what has been called the golden era of medical bacteriology. Between 1879 and 1889 German microbiologists isolated the organisms that cause cholera, typhoid fever, diphtheria, pneumonia, tetanus, meningitis, and gonorrhea as well the Staphylococcus and Streptococcus organisms. Even as Koch's work was influencing the development of the germ theory, the influence of the English physician Joseph Lister (1827-1912) was being felt in operating rooms. Building on the work of both Semmelweis and Pasteur, Lister—for whom the well-known antiseptic mouthwash Listerine was named—began soaking surgical dressings in carbolic acid, or phenol, to prevent postoperative infection.
Whereas antisepsis was the great battleground of the invisible war during the nineteenth century, in the twentieth century the most important struggle concerned the development of antibiotics. The first effective medications to fight bacterial infection in humans were sulfa drugs, developed in the 1930s. They work by blocking the growth and multiplication of bacteria and were initially effective against a broad range of bacteria, but many strains of bacteria have evolved resistance to them. Today, sulfa drugs are used most commonly in the treatment of urinary tract infections and for preventing infection of burn wounds.
The importance of sulfa drugs was eclipsed by that of penicillin, first discovered in 1928 by the British bacteriologist Alexander Fleming (1881-1955). Working in his laboratory, Fleming noticed that a mold that had fallen accidentally into a bacterial culture killed the bacteria. Having identified the mold as the fungus Penicillium notatum, Fleming made a juice with it that he called penicillin. He administered it to laboratory mice and discovered that it killed bacteria in the mice without harming healthy body cells.
It would be more than a decade before the development of a form of penicillin that could be synthesized easily. This drug arrived on the scene in 1941—just in time for the years of heaviest fighting in World War II—and after the war pharmaceutical companies began to manufacture numerous varieties of antibiotic. By the last decade of the twentieth century, however, a new problem emerged: bacteria were becoming resistant to antibiotics. This has been the case with medications used to treat conditions ranging from children's ear infections to tuberculosis.
An example is amoxicillin, a penicillin derivative developed in the late twentieth century. Many pediatricians found it a better treatment than penicillin for ear infections, because it did not tend to cause allergic reactions sometimes associated with the other antibiotic. However, by the late 1990s evidence surfaced indicating that certain types of bacteria had developed a protein that rendered amoxicillin ineffective against ear infections. Critics of amoxicillin (or of antibiotic treatments in general) maintained that widespread prescription of the antibiotic actually helped create that situation, because the bacteria developed the protein mutation defensively. Because of these and similar concerns associated with antibiotics, doctors have begun taking measures toward controlling the spread of antibiotic-resistant diseases, for instance by prescribing antibiotics only when absolutely necessary. Research into newer types and combinations of drugs is ongoing, as is research regarding the development of vaccines to prevent bacterial infections.
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A substance produced by or derived from a microorganism, which in diluted form is capable of killing or at least inhibiting the action of another microor ganism. Antibiotics are not usually effective against viruses.
The practice of inhibiting the growth and multiplication of microorganisms, generally by ensuring the cleanliness of the environment.
An area of the bio logical sciences concerned with bacteria, including their importance in medicine, industry, and agriculture.
Deoxyribonucleic acid, a molecule in all cells, and many viruses, containing genetic codes for inheritance.
A term for an infection that occurs when the body's resistance is lowered. Compare with exogenous.
Affecting or potentially affecting a large proportion of a popula tion (adj.) or an epidemic disease (n.)
An area of the medical sciences devoted to the study of disease, including its incidence, distribution, and control within a population.
A branch of medical study concerned with the causes and origins of disease; also, a general term referring to all the causes of a particular disease or condition.
A term for an infection that originates outside the body. Compare with endogenous.
A unit of information about a particular heritable (capable of being inherited) trait that is passed from parent to offspring, stored in DNA molecules called chromosomes.
A theory in medicine, widely accepted today, that infections, contagious diseases, and other conditions are caused by the actions of microorganisms.
The condition of being able to resist a particular disease, particularly through means that prevent the growth and development of pathogens or counteract their effects.
The study of the immune system, immunity, and immune responses.
A state or condition in which parasitic organisms attach them selves to the body or to the inside of the body of another organism, producing contamination and disease in the host.
The prevention of a disease by the introduction to the body, in small quantities, of the virus or other microorganism that causes the disease.
Alteration in the physical structure of an organism's DNA, resulting in a genetic change that can be inherited.
Affecting an extremely high proportion of a population over a wide geographic area (adj.) or a disease of pandemic proportions (n.)
A general term for any organism that depends on another organism for support, which it receives at the expense of the other organism.
A biological discipline devoted to the study of parasites, primarily those among the animal and protist kingdoms. Parasitic bacteria, fungi, and viruses usually are studied within the context of infectious diseases.
A disease-carrying para site, typically a microorganism.
The study of the essential nature of diseases.
A set of policies and methods for protecting and improving the health of a community through efforts that include disease prevention, health education, and sanitation.
Ribonucleic acid, the molecule translated from DNA in the cell nucleus, the control center of the cell, that directs protein synthesis in the cytoplasm, or the space between cells.
A preparation containing microorganisms, usually either weakened or dead, which is administered as a means of increasing immunity to the disease caused by those microorganisms.
An organism, such as an insect, that transmits a pathogen to the body of a host.