When parasites cause a disease they are known as pathogens. Disease caused by a pathogen is called infection. Who and what are these infective agents? How, being surrounded by them, do we defend ourselves from continous infections? Under what conditions can pathogens breach our defenses and give rise to an infection? And what means do we have to overcome infections once they have started/

Pathogens

In order of size, from smallest to largest, pathogens are classified as follows:

Viruses: subcellular, semiliving particles

Bacteria: unicellular, plantlike organisms

Fungi: unicellular and multicellular plantlike organisms

Protozoa: unicellular animals

Parasitic worms: multicellular animals

Viruses

Viruses, the smallest of the pathogens, are on the borderline between living and nonliving matter. They are visible only with an electron microscope. The pure virus particle appears to have no metabolism of its own. Viruses lack all the enzymes essential to energy production and protein synthesis in normal animal cells, and they cannot grow or reproduce by themselves. They must lead a parasitic sxistence inside a cell, borrowing what they need for growth and reproduction from the cells they invade. Once a virus is inside the host cell, its genetic material takes control of the cell and tricks it into manufacturing more viruses like itself.

The normal functioning of the host cell is thereby disrupted. Different viruses infect different kinds of cells, and the seriousness of the disease they cause depends greatly on which kind of cell is infected. The viruses that causes colds, for example, attack upper respiratory tract cells, which are constantly cast off and replaced. The disease is therefore mild. Polio virus, in contrast, attacks nerve cells that cannot be replaced, and the consequences, such as paralysis, are severe.

Over 150 viruses are know to cause disease in humans. Illnesses caused by viruses are the most common forms of contagious disease. They include most of the minor ailments that cause transitory illness and are rarely precisely diagnosed.

Among these are the common cold; a variety of undiagnosed, short-lived respiratory infections; influenza; gastrointestinal upsets that cause diarrhea and can last for only 24 hours; and assorted aches and pains. More serious are the diseases that occure mainly in childhood and frequently cause a severe rash, such as measles, chickenpox, and mumps. Smallpox, which used to be the most severe of these diseases, has now been eliminated thanks to an encompassing vaccination program carried out by the World Health Organization in the 1960s and 70s.

Infections from a variety of herpes simlpex virus (type I) lead to sores that last from a few days to a week or two and appear mainly on the face around the mouth. ( Herpes of the eye can be extremely serious and leads to loss of vision). A second kind of herpes virus (type II) gives rise to a painful genital disease of similar duration. More severe infections caused by viruses aare poliomyelitis, a disease of the nervous system, and hepatitis.

There are two major kinds of hepatitis. One is mainly transmitted by hypodermic injection and is common among drug addicts (hepatiitis B). The latter is particularly prevalent in areas with poor hygienic conditions, however, it can also be spread readily in societies with high hygienic standards via water contamination with sewage efflux. It can be contracted on contaminated beaches, and other swimming areas close to sewage outlets. The disease can be mild, although it is generally debilitating and requires a long period of bed rest. If neglected, hepatitis can lead to extensive liver damage and debilitation of the organism that can cause other infections to appear. In a similar category is the disease of children and young adults-infectious mononucleosis. This is a virus infection that affects white blood cells and gives rise to a deblitated condition.

With adequate rest, there are no consequences; however, when neglected, the spleen may rupture, causing the need for removal of this organ and a life-long increase in susceptibility to infection. Certain viruses can also cause proliferation of cells. The human disease cloearly based on this property are warts, which is generally a very mild disease , nonetheless, can become extremely severe if the resistance of the infected invidual breaks down. In rodents many viruses can cause cancer, and they are suspected of doing so in humans, but this has not yet been demonstrated.

Bacteria

Bacteria, which can be seen under a light microscope, are generally 100 to 1,000 times larger than viruses. On the other hand, they are considerable smaller than mammalian cells, most of which can hold 50 to 100 bacteria. Of several thousand species of bacteria, approximately 100 cause disease in humans. Most bacteria are nither parasites nor pathogens, however, and many are quite beneficial. Unlike viruses, many, but not all disease-causing bacteria do not have to enter cells to cause infection. They generally thrive on, around, and between human cells and can cause harm through the toxins and poisonous enzymes that they produce. (Some bacterial toxins are among the most powerful poisons known. One fourth of a teaspoon of a toxin released by the tetanus organism can kill over 10,000 people).

Some of the toxins and poisonous enzymes bacteria produce work locally, killing and dissolving the cells near the site of the infection. The damaged cells become food for the bacteria. The infection then spreads deeper into the surrounding tissue, giving rise to boils, abscesses, soreness, and the like. Other bacterial products are carried by the bloodstream, causing fever or affecting vital organs and systems (such as the nervous system in a tetanus infection). Some pathogenic bacteria have no specific toxic attributes. They grow to large numbers and cause damage by obstructing vital pathways of the host: the lungs, for example, and by consuming materials needed by host cells for sustenance.

Generally speaking, almost all bacterial infections require attention. Chief among these are skin infections such as small abscesses or boils, which, when unattended, can give rise to major complications. Such superficial skin infections are generally caused by bacteria known as staphylococci and streptococci. If the infection persists, it may cause a more serious disease known as impetigo, which requires a physicians attention. Acne is caused by relatively innocuous bacteria growing in hair follicles (pores that give rise to hair). These bacteria plug the pores and produce inflammatory substances that caus pus formation. Good hygiene often can remove sebum and other substances that such bacteria require.

Common diseases of the respiratory pathway are strep throat, bronchitis, and pneumonia. These vary in severity: because they generally respond rapidly to treatment with antibodies, they should be treated early. Tuberculosis, a disease of the lungs, is caused by a very resistant bacterium and used to be a major problem. Sufferers had to be isolated from the rest of the population during the infectious stage of the disease. The incidence of tuberculosis has significantly declined through the use of certain drugs. Nonetheless, the disease is far from being eradicated even in the the United States, and it still presents a major problem in many countries of the world.

Periodontal disease, an infection of the gum, attacks over 50 percent of the population. It is caused by a variety of bacteria and can lead to extensive and painful consequences. Probably the most common consequence of bacterial infection is dental caries, a scourge known to almost everyone who has ever sat in a dentist's chair. It is caused by products of streptococci, which are related to the organism that cause strep throat.

Bacterial diseases of the stomach and intestines (gastrointestinal or G.I. tract) are more common in children than in adults. However, localized G.I. infections such as appendicitis can occur at any age. In middle-age and older people another localized form of infection, known as diverticulitis, can occur. This results from food caught in little deformations of the intestine known as diverticuli. Food poisoning is not always a consequence of direct infection. It can be caused by food that has been standing at temperatures that allow bacteria to grow. These bacteria produce toxins that can cause severe illness and high fever, and it is wise not to eat food that has been standing in a warm place for more than six hours.

Among foods that can give rise to severe infection are insufficiently cooked poultry, unwashed eggs, and occasionally, prepared meats, such as sausage. Infections arising from contaminated food are caused by organism called salmonella and are therefore known as salmonellosis. The most lethal bacterial poison carried by food is that of the botulinum organism. These organism grow in the absence of air, and improperly canned food is the main source of them. Home canned foods are the chief offenders. The early appearing symptoms of botulism, is weakness, dizziness, and nausea soon give way to respiratory paralysis, which can lead to death within 36 or 48 hours after food consumption.

Sterilizing foods by cooking prior to canning avoids the growth of the organism. The toxin itself is degraded by boiling the canned food for 10 minutes. The urinary tract of women are particullarly accessible to bacteria, and urine presents a relatively good source of bacterial nutrient. Therefore, bladder infections occur fairly frequently in women. Although these infections are not necessarily severe, they can become so if they are not treated promptly. Such infections can also lead to an ascending infection of the kidneys, which can be extremely serious.

Some bacteria are like viruses in that they can reproduce only within living host cells. A groupe of these called rickettsiae, are usually transmitted by insects such as fleas, lice mites, and ticks. Diseases that are caused by rickettsiae include typhus fever, and Rocky Mountain spotted fever. Another of bacterium that needs the host cell causes an eye infection called trachoma. The trachoma organism is spred by poor hygienic conditions, and it affects over 200 million people in Asia and Africa. In the United States the chief infection due to the trachoma organism (thought to be chlamydia trachomatis)is sexually transmitted disease in women. This disease is difficult to diagnose. it has reached nation wide epidemic proportions, rivaling gonorrhea.

Fungi

Fungi are primitive plants that may be multicellular (like molds) or unicellular ( like yeasts). Mushrooms and the molds that form on bread and cheese are all examples of fungi. Only about 50 fungi of many thousands of species cause disease in humans, and these diseases usually are restricted to the skin, mucous membranes, and lungs. Curing fungal disease is extremely difficult. To defend against treatments, some fungi form spores, which are an especially resistant dormant stage of the organism.

The most common fungal malady is candidiasis, a yeast infection of the vagina that can also occur in other areas of the body, especially in the mouth in infants, and (is known as trush). Candidiasis, a relatively mild disease that causes itching, should be examined by a physician. When unattended and persistent, this disease can become severe and involve inflammation of the mucous membranes on which it normally exist. Another common group of fungal diseases are diseases of the skin, including athlete's foot, jock itch, and ringworm, a disease of the scalp. These three are rather mild diseases that, although difficult to treat, rarely give rise to major problems. Fungi can also cause systemic (infecting large portions of the body). Such disease is severe, life-threatening, and extremely diddicult to treat. Among the systemic forms of fungal disease are cryptococcosis and coccidioidomycosis, also known as valley fever because it is most frequently seen in the San Joaquin Valley, here in California, the U.S.A.

Protozoa

The protozoa, microscopic single-celled animals, are associated with such tropical diseases as malarir, African sleeping sickness, and amoebic dysentery. Many protozoa-based diseases are recurrent. The pathogen remains in the body, alternating between activity and inactivity. Hundreds of millions of Asians, Africans, and South Americans suffer from protozoal infections. The most common protozoal disease in the United States is trichomoniasis, a vaginal infection.

Parasitic Worms

The parasitic worms are the largest organisms that can enter the body to cause infection. The tapeworm, i.e, can grow to a length of several feet. Worms, including such intestinal parasites as the tapeworm, hookworm, and pinworm, cause a large variety of relatively mild infections. Smaller worms, known as flukes, infect organs such as the liver, and lung, and can be quite deadly.

Diagnosis of Pathogenic Diseases

Although most fungal and protozoal diseases are relatively easy to diagnose as such, it is frequently difficult to distinguish viral and bacterial diseases. In general, however, the most common viral diseases are relatively mild and of short duration. The symptoms they produce are usually caused by the reactions of defense systems of the body trying to eliminate the invading virus. Because this defense requires elimination of an invader that inhabits the body's own cells, some tissue destruction results. The severity of the disease is frequently dependent on the extent of the body's own response. Such tissue destruction is often accompanied by fever. In addition, reflexes such as mucus secretion and coughing, which serve to remove the invading organism, are perceived as noxious by the suffering patient. Fortunately, mild infections are typically of short duration.

The more common bacterial infections, on the other hand, last longer and are usually accompanied by some marked symptoms. There are notable exceptions, however. The most important exception is gonorrhea, which, especially in women, may show no symptoms for long periods of time but remain infectious, and capable of causing severe problems. The hallmark of bacterial infections, when they are localized, is the accumulation of pus, which is seldom present with viral infections.

The common fungal and protozoal infections, especially those prevalent in the Western world, are generally marked by their persistence, and their mildness. It must be remembered, however, that any mild condition can become severe when it is neglected and allowed to spread or when the defense that keep it limited break down. It is a sound health principle that any persistent infection condition, even if it is mildly disturbing to the invidual, should be treated.

Body Defenses

The primary lines of defense of the body are the external barriers that