HUMAN DISEASES (Part 1 of 7)

A disease is a condition that impairs the proper function of the body or of one of its parts.

Every living thing, both plants and animals, can succumb to disease. People, for example, are often infected by tiny bacteria, but bacteria, in turn, can be infected by even more minute viruses.

Hundreds of different diseases exist. Each has its own particular set of symptoms and signs, clues that enable a physician to diagnose the problem. A symptom is something a patient can detect, such as fever, bleeding, or pain. A sign is something a doctor can detect, such as a swollen blood vessel or an enlarged internal body organ.

Every disease has a cause, although the causes of some remain to be discovered. Every disease also displays a cycle of onset, or beginning, course, or time span of affliction, and end, when it disappears or it partially disables or kills its victim.

Endemic disease (also called childhood disease), disease continually prevalent in a region.

An epidemic disease is one that strikes many persons in a community. When it strikes the same region year after year it is an endemic disease.

An acute disease has a quick onset and runs a short course. An acute heart attack, for example, often hits without warning and can be quickly fatal. A chronic disease has a slow onset and runs a sometimes years-long course. The gradual onset and long course of rheumatic fever makes it a chronic ailment.

How Germs Invade the Body

Humans live in a world where many other living things compete for food and places to breed. The pathogenic organisms, or pathogens, often broadly called germs, that cause many diseases are able to invade the human body and use its cells and fluids for their own needs. Ordinarily, the body's defence system can ward off these invaders.

Pathogenic organisms can enter the body in various ways. Some such as those that cause the common cold, pneumonia, and tuberculosis are breathed in. Others such as those that cause venereal diseases enter through sexual contact of human bodies. Still others such as those that cause bacillary dysentery, cholera, and typhoid fever get in the body through contaminated food, water, or utensils.

Insects can spread disease by acting as vectors, or carriers. Flies can carry germs from human waste or other tainted materials to food and beverages. Germs may also enter the body through the bite of a mosquito, louse, or other insect vector.

Kinds of Disease

Infectious, or communicable, diseases are those that can be passed between persons such as by means of airborne droplets from a cough or sneeze. Tiny organisms such as bacteria and fungi can produce infectious diseases. So can viruses. So can tiny worms. Whatever the causative agent might be, it survives in the person it infects and is passed on to another. Or, its eggs are passed on. Sometimes, a disease-producing organism gets into a person who shows no symptoms of the disease. The asymptomatic carrier can then pass the disease on to someone else without even knowing he has it.

Non-infectious, or non-communicable, diseases are caused by malfunctions of the body.

These include organ or tissue degeneration, erratic cell growth, and faulty blood formation and flow. Also included are disturbances of the stomach and intestine, the endocrine system, and the urinary and reproductive systems. Some diseases can be caused by diet deficiencies, lapses in the body's defence system, or a poorly operating nervous system.

Disability and illnesses can also be provoked by psychological and social factors. These ailments include drug addiction, obesity, malnutrition, and pollution-caused health problems.

Furthermore, a thousand or more inheritable birth defects result from alternations in gene patterns. Since tiny genes are responsible for producing the many chemicals needed by the body, missing or improperly operating genes can seriously impair health. Genetic disorders that affect body chemistry are called inborn errors of metabolism. Some forms of mental retardation are hereditary.

HOW THE BODY FIGHTS DISEASE

Mucous membrane (or mucosa), membrane that secretes mucus and lines the mouth, nose, throat, windpipe, lungs, eyelids, and the alimentary canal.

As a first line of defence, a healthy body has a number of physical barriers against infection. The skin and mucous membranes covering the body or lining its openings offer considerable resistance to invasion by bacteria and other infectious organisms. If these physical barriers are injured or burned, infection resistance drops. In minor cases, only boils or pimples may develop. In major cases, however, large areas of the body might become infected.

Cilia (plural of cilium), hairlike, vibratory appendages found in some plants and animals.

Breathing passages are especially vulnerable to infection. Fortunately, they are lined with mucus-secreting cells that trap tiny organisms and dust particles. Also, minute hairs called cilia line the breathing passages, wave like a field of wheat, and gently sweep matter out of the respiratory tract. In addition, foreign matter in the breathing passages can often be ejected by nose blowing, coughing, sneezing, and throat clearing. Unfortunately, repeated infection, smoking, and the repeated use of strong chemicals (including alcohol and drugs) can damage the respiratory passageways and make them more susceptible to infection.

Scavenger cells are present too in the walls of the bronchi, the branched air tubes to the lungs. Foreign matter reaching the bronchi after evading the other defences can be "eaten" by the scavengers and disposed of in the lymph glands of the lungs.

Many potential invaders cannot stand body temperature (98.6° F or 37° C). Even those that thrive at that temperature may be destroyed when the body assumes higher, fever temperatures.

Wax in the outer ear canals and tears from eye ducts can slow the growth of some bacteria. And stomach acid can destroy certain swallowed germs.

Lymph, a colourless liquid exuded through capillaries to nourish tissues of the body.

The body's second line of defence is in the blood and lymph. Certain white blood cells flock to infected areas and try to localize the infection by forming pus-filled abscesses.

Unless the abscess breaks and allows the pus to drain, the infection is likely to spread. When this happens, the infection is first blocked by local lymph glands. For example, an infection in the hand travels up the arm, producing red streaks and swollen, tender lymph glands in the armpit. Unless the infection is brought under control, it will result in blood poisoning.

Scavenger cells, or phagocytes, are located at various sites to minimize infection. One type in the spleen and liver keeps the blood clean. Others in such high-risk areas as the walls of the bronchi and the intestines remove certain bacteria and shattered cells.

How We Become Immune to Disease

The body has a special way of handling infection. It has a system that fends off the first traces of an infectious substance and then, through a "memory," gives the body a long-lasting immunity against future attacks by the same kind of invader.

Antigen, a substance in blood that causes production of antibodies against itself.

Antibody, the protective substance produced in body fluids in response to exposure to foreign antigen in blood.

Many substances could harm the body if they ever entered it. These substances, or antigens, range from bacteria and pollen to a transplanted organ (viewed by the body as an invader). To fight them the body makes special chemicals known as antibodies.

Antibodies are a class of proteins called immuno-globulins. Each antibody is made of a heavy chain of chemical subunits, or amino acids, and a light chain of them. The light chain has special sites where the amino acids can link with their complements on the antigen molecule. When an antibody hooks up with an antigen, it often puts the antigen out of action by inactivating or covering a key portion of the harmful substance. In some cases, through the process of opsonization, antibodies "butter" the surface of some antigens and make them "tastier" to phagocytes, which engulf the antigens. Sometimes an antibody hooks to a bacterial antigen but needs an intermediate, or complement, to actually destroy the bacterium. As the antibody-antigen complex circulates in the blood, the complex "fixes" complement to it. In turn, the complement causes powerful enzymes to eat through the bacterial cell wall and make the organism burst.

There are several kinds of immuno-globulins IgM, the largest; IgG, the most plentiful and versatile; and IgA, the next most plentiful and specially adapted to work in areas where body secretions could damage other antibodies. Other immuno-globulins are tied in with allergic reactions. IgM is made at the first signs of an antigen. It is later supplanted by the more effective IgG.

When infection first strikes, the immunity system does not seem to be working. During the first day or so, antibodies against the infection cannot be found in the blood. But this is only because the basic cells involved in antibody production have been triggered by the presence of antigen to multiply themselves. The antibody level starts to rise on about the second day of infection and then zooms upward. By the fifth day the antibody level has risen a thousandfold.

The first antibodies, the large IgM type, are not the best qualified to fight a wide range of antigens, but they are particularly effective against bacteria. The more versatile IgG is circulating in the blood on about the fourth day of infection. Its production is stimulated by the rising level of IgM in the blood. At this time, IgM production drops off and the immunity system concentrates on making IgG. The IgG type of antibody sticks well to antigens and eventually covers them so that the antigens can no longer stimulate the immune response and IgG production is switched off. This is an example of negative feedback control.

Antibody Production

Thymus, organ, located behind the breastbone and above the heart; participates in the production of white blood cells or lymphocytes; because it attains maximum size at puberty and becomes smaller in adults, researchers feel it may be an endocrine gland that affects growth and sexual maturation.

Antibodies are made by two kinds of cells plasma cells and a class of white blood cells, lymphocytes. Plasma cells actually originate from lymphocytes and are found throughout the lymphatic tissue. Lymphocytes stem from cells in the blood-forming sections of bone marrow. When the bone-marrow cells circulate to the thymus, a lymphatic structure in the chest, they receive "orders" to become lymphocytes and make antibodies. Most lymphocytes last for only a few hours, but a few wander through the blood and body tissues for years. These lymphocytes are responsible for "remembering" old antigens and for inducing the immunity system to produce antibodies against those or similar antigens if they ever again enter the body.

When people develop antibodies against a disease by the action of their own immunity system, they have active immunity. When they are given someone else's antibodies, however, they just have passive immunity to a disease.

Passive immunity is only temporary. Some people may also get temporary relief from a disease through injections of serum containing gamma globulin, a portion of the blood rich in antibodies.

Without protective antibodies, we could die of the first disease that struck us. This would be true, too, of newborn babies, except that they receive passive immunity from their mothers. During her lifetime, a mother accumulates a wide variety of antibodies against a host of diseases. Enough of them are passed to the developing baby in her womb to give it a temporary immunity to many diseases during the early months of its life, until it can develop its own set of antibodies.