W HAT I S I NFECTION ?

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.

S OME T ERMS

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.

• Bacteriology: An area of the biological sciences concerned with bacteria, including their importance in medicine, industry, and agriculture)
• Epidemiology: An area of the medical sciences devoted to the study of disease, including its incidence, distribution, and control within a population)
• Etiology: 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)
• Immunology: The study of the immune system, immunity, and immune responses)
• Pathology: The study of the essential nature of diseases)
• Virology: The study of viruses

In addition, there are several terms relating to the prevention of infection.

• Antibiotic: 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 microorganism. Antibiotics typically are not effective against viruses.
• Antisepsis: The practice of inhibiting the growth and multiplication of microorganisms)
• Germ theory: A theory in medicine, widely accepted today, that infections, contagious diseases, and other conditions are caused by the actions of microorganisms)
• Immunity: A condition of being able to resist a particular disease, particularly through means that prevent the growth and development of pathogens or counteract their effects
• Inoculation: The prevention of a disease by the introduction to the body, in small quantities, of the virus or other microorganism that causes the disease
• Vaccine: A preparation containing microorganisms, usually either weakened or dead, which are administered as a means of increasing immunity to the disease caused by those microorganisms

Some of these words appear in this essay and others in related essays on infectious diseases and immunity.

B ACTERIA

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.

V IRUSES

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.