The human body is both surrounded and inhabited by billions of microorganisms (living organisms that are so small that they can be seen only with a microscope). Most microorganisms are harmless or even beneficial; for example, bacteria that normally live in the digestive system help digest food.
Occasionally, however, a microorganism capable of causing a disease invades the body. Diseases caused by such microorganisms are called infectious diseases.
Infectious diseases are contagious; that is, they can be passed from one person to another. They can be transmitted by skin contact, through body fluids, in contaminated food or drink, or via airborne particles containing the microorganisms, although the pathways and ease of transmission vary by disease.
Animal or insect bites are another means of transmission. (If an insect, for example, bites an infected person, the insect can carry the microorganism and pass the disease by biting another person.) The two most common types of infectious diseases are bacterial infections and viral infections.
Disease-causing, or pathogenic, bacteria either attack the body's tissues directly or cause damage by secreting poisonous substances called toxins. Fortunately, bacterial infections are often curable. Certain bacteria can be killed by drugs; other bacterial diseases can be prevented by vaccination.
Viruses are the smallest known microorganisms. They are responsible for diseases as relatively harmless as the common cold and as serious as meningitis. Viruses live and reproduce only within living cells, and only certain cells are susceptible to a specific virus. You can be host to many viruses without suffering any adverse effects, but if enough cells are attacked, you will become sick.
There is no effective medical treatment for most viral infections. Because a virus lives inside a cell, any treatment designed to kill the virus is also likely to harm the cell. In addition, there are thousands of different viruses -- each one with different properties -- and an agent effective against one virus probably will not affect the others. Although there are vaccinations to protect against some viral diseases, therapy for most viral diseases is limited to treating the symptoms.
In this article, we'll focus on the many facets of infectious diseases, starting with how the body defends against them.
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The Body's Defenses
Despite the prevalence of disease-causing microorganisms, the body is not defenseless against these invaders. The body fights infections in three ways: by preventing the organisms from entering the body, by attacking those that do manage to enter, and by inactivating those organisms it cannot kill.
Sometimes, too, the body fights disease by developing defensive symptoms. Fever is an example. During an illness, the body's temperature regulator may respond to the illness by raising the body's temperature. Some researchers believe that this is an effective response because the microorganisms causing the disease may not be able to survive the higher body temperature.
The skin is the first barrier that guards the underlying tissues of the body. Where there are natural openings in the skin, there are also defenses. For example, tear glands in the eyes secrete and bathe the eyes with fluid that contains bacteria-fighting components. The salivary glands in the mouth and the tonsils in the throat help prevent microorganisms from attacking the mouth and throat.
Many openings, as well as internal passages, in the body are lined with mucous membranes. These delicate layers produce mucus, a slippery secretion that moistens and protects by repelling or trapping microorganisms.
Internally, certain body organs fight infection. For instance, the liver and the spleen (a large glandlike organ located in the abdomen) filter out harmful substances from the blood flowing through them. The lining of the stomach produces acids that attack germs in food that has been eaten. The body's lymph system manufactures white blood cells, which attack and kill invading organisms.
Now let's get even more specific in our look at the body's defenses. We'll start by describing the lymph system.
The Lymph System
The lymph system is a network of vessels that carry lymph, a watery fluid containing white blood cells, throughout the body. Lymph drains from the blood vessels and body tissues, carrying away waste products. The waste products are filtered out of the lymph by small structures called lymph nodes. Within the lymph nodes, harmful microorganisms are trapped, attacked, and destroyed by white blood cells. This is one of the body's primary and most efficient lines of defense.
Antibodies are manufactured in the lymph system. Antibodies are protective substances that the body produces in response to invasion by a hostile organism or the presence of a foreign substance. Antibodies counteract some invading bacteria and viruses by inactivating them so that they are powerless. Antibodies that neutralize toxins (poisons) produced by bacteria are called antitoxins.
The body's production of white blood cells and antibodies in response to an invading organism is called the immune reaction. Immunity is the body's ability to resist an invasion of disease-causing bacteria and viruses. Once antibodies have been made to fight a certain type of microorganism, that microorganism usually no longer poses a threat to the body. That is why one attack of a disease often prevents its recurrence down the road. The first attack causes antibodies to be produced, and these antibodies protect the system against future attacks.
There are ways to help the body's own defenses work. One is immunization, something all of us have experience with. Read about it in the next section.
Immunity can be provided artificially by vaccination and other forms of immunization. A vaccine is a preparation containing the offending organism -- usually in a weakened form that will not cause the actual disease. When introduced into the body, the vaccine stimulates the body to produce antibodies against the disease. These antibodies often remain in the system for life, and the body is thus prepared to resist the actual disease.
A number of viral diseases can be prevented by immunization. There are vaccines for polio, measles, rubella (German measles), mumps, some strains of influenza, and chicken pox. A vaccine against the organism Hemophilus influenzae also is available. This vaccine prevents the most common cause of bacterial meningitis in children.
If immunization aims to confer long-term resistance to certain maladies, medications generally are a more immediate defense. We will cover medication in the next section. Read on.
Besides vaccination, there are other measures that can be used to help the body's natural defenses fight off infectious diseases. Bacterial infections can often be conquered by medications called antibiotics. These are substances derived from living microorganisms that kill other microorganisms.
Penicillin, for example, comes from a living mold called Penicillium. Penicillin is one of the most commonly used antibiotics, along with cephalosporins, tetracycline, and erythromycin. Each of these is effective against specific diseases. Antibiotics work either by destroying bacteria or by preventing their reproduction. Sulfonamides, or sulfa drugs, are synthetic antibiotic drugs that also are effective against infections. This type of drug is often prescribed for localized infections in the urinary tract.
Unfortunately, these medications do not attack viruses. Viruses, therefore, are responsible for many of the serious or fatal infectious illnesses today because no generally effective cures have been developed. The virus that causes acquired immunodeficiency syndrome (AIDS) is an example of a virus for which there is no cure or vaccine.
Researchers are continually searching for new ways to help the body combat infectious diseases. Medical advances against these diseases have already been dramatic; not too many years ago, infectious diseases were uncontrollable and thus a constant danger. The control of many infections has been one of medicine's greatest accomplishments.
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