Understanding Infections

BACKGROUND

 

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.

 

DEFINITIONS

 

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 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.
  • endogenous infection, 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 behaviours tend to be coupled with poor eating habits, which invite infection by denying the body the nutrients it needs.
  • microorganism or pathogen, the germ itself
  • 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)

 

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.

 

Five major groups of microorganisms are responsible for the majority of infections:

 

  • Protozoa
  • Helminths, or worms, both of which are considered in Parasites
  • Bacteria
  • Viruses  
  • Fungi

 

In the present context viruses and bacteria are inquired.

 

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 and are known as

-          vibrio, bacteria having a shape like that of a comma.

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.

 

  • Viruses

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.

 

BACTERIA AND HUMANS

 

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.

 

Bacterial Infections

 

Many bacteria attack the:

 

 

 

Viral Infections

 

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.

 

hepatitis

chicken pox

polio

measles

rabies.

Cold and flue

Orthomyo infections

Pneumonia

AIDS   is one famous infection generated by retrovirus

 

 

ANTIBIOTICS

 

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. 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, 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

 

 

DISCUSSION

 

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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