Development of Infection: Biology of Infectious Disease: Merck Manual Home Edition

THE MERCK MANUAL MEDICAL LIBRARY: The Merck Manual of Medical Information--Home Edition

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	Development of Infection

Section		Infections

Subject		Biology of Infectious Disease

Topics		Introduction· Defenses Against Infection·Development of Infection· Infections in People With Impaired Defenses· Prevention of Infection· Resident Flora

Development of Infection

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Pronunciations

polymerase chain reaction

salmonella

tularemia

Infectious diseases are usually caused by microorganisms that invade the body and multiply. Invasion by most microorganisms begins when they adhere to a person's cells. Adherence is a very specific process, involving "lock-and-key" connections between the microorganism and cells in the person's body. Whether the microorganism remains near the invasion site or spreads to other sites depends on such factors as whether it produces toxins, enzymes, or other substances.

Some microorganisms that invade the body produce toxins. For example, Clostridium tetani in an infected wound produces a toxin that causes tetanus. Some illnesses are caused by toxins produced by microorganisms outside the body. Food poisoning caused by staphylococci is one example. Most toxins contain components that bind specifically with molecules on certain cells (target cells). Toxins play a central role in such diseases as tetanus, toxic shock syndrome, botulism, anthrax, and cholera.

After invading the body, microorganisms must multiply to produce infection. After multiplication begins, one of three things can happen: the microorganisms can continue to multiply and overwhelm the body's defenses; a state of balance can be achieved, producing a chronic infection; or the body—with or without medical treatment—can destroy and eliminate the invading microorganism.

Many disease-causing microorganisms have properties that increase the severity of the diseases they cause (virulence) and help them resist the body's defense mechanisms. For example, some bacteria produce enzymes that break down tissue, allowing the infection to spread faster.

Some microorganisms have ways of blocking the body's defense mechanisms. For example, a microorganism may be able to interfere with the body's production of antibodies or T cells (a type of white blood cell) specifically armed to attack them. Others have outer coats (capsules) that resist being ingested by white blood cells. The fungus Cryptococcus actually develops a thicker capsule after it enters the lungs for the specific purpose of resisting the invaded body's defenses. Some bacteria resist being split open (lysed) by substances circulating in the bloodstream. Some even produce substances that counter the effects of antibiotics.

Identifying an Infectious Organism

It is usually important to know what specific microorganism is causing an illness. Many different microorganisms can cause a given condition (for example, pneumonia can be caused by viruses, bacteria, or fungi), and the treatment is different for each organism.

There are many ways to identify microorganisms. Despite the development of rapid identification systems, direct microscopic examination of samples taken from the site of infection is often the most rapid method of identifying microorganisms capable of causing disease. But the microorganisms must be of sufficient size and number to be seen with a regular microscope. Sometimes microorganisms can be seen with a microscope and recognized by characteristic shapes and colors. Usually, however, the microorganisms are too few or too small to see, so they may be grown in the laboratory until there are enough to be recognized with chemical tests. The process of growing the organism is called a culture. Many microorganisms can be grown this way; such as the bacteria that cause gonorrhea and strep throat. Cultures can also be used to test the sensitivity of microorganisms to various antibiotics, which can help a doctor determine what drug to use in treating an infected person. This strategy is particularly important because microorganisms are constantly developing resistance to antibiotics that were previously effective.

Some microorganisms, such as the bacterium that causes syphilis and the virus that causes AIDS, are very difficult to culture. These infections, and many others, can be identified by finding antibodies to the microorganisms in the infected person's blood or body fluids (for example, cerebrospinal fluid). Antibody- based tests are used to identify many infections, but they are not always reliable. Antibodies often stay in the body for many years after an infection has gone away. New tests, such as the polymerase chain reaction (PCR), identify pieces of the microorganism's genetic material (DNA), which are only present when the organism is present.

These tests are performed only when a doctor already suspects a particular disease. Therefore, a doctor's understanding of all the features of a person's illness, including symptoms, physical examination, and risk factors, is essential for diagnosing an infection.

Biological Warfare and Terrorism

Biological warfare is the use of microbiological agents for hostile purposes. Such use is contrary to international law and in fact has rarely taken place during formal warfare in modern history, despite the extensive preparations and stockpiling of biological agents carried out in the 20th century by most major powers. Currently, the NATO nations have taken biological weapons out of service. Some other nations (including Iran, and North Korea) are thought to maintain biological warfare capability. For a variety of reasons—including uncertain military efficacy and the threat of massive retaliation—experts consider the use of biological agents in formal warfare unlikely. However, biological agents are thought by some people to be an ideal weapon for terrorists. These agents may be delivered clandestinely, and they have delayed effects—allowing the user to remain undetected.

Potential biological agents include anthrax, botulism toxin, brucellosis, encephalitis viruses, hemorrhagic fever viruses (Ebola and Marburg), plague, tularemia, and smallpox. Each of these is potentially fatal and, except for anthrax and botulism toxin, can be passed from person to person. Anthrax spores are relatively easy to prepare and, unlike most other agents, can be spread through the air, creating the potential for distribution by airplane. Theoretically, 1 kilogram of anthrax could kill 10,000 people, although technical difficulties with preparing the spores in a sufficiently fine powder would probably limit actual deaths to a fraction of this.

Despite these theoretical concerns, the only successful terrorist use of anthrax—multiple pieces of contaminated mail delivered to a variety of locations in the United States in 2001—resulted in only a handful of deaths and serious infections. A larger number of people were contaminated with anthrax spores without developing illness, possibly because of extensive use of the antibiotic ciprofloxacin Some Trade Names
CILOXAN
CIPRO
. However, there was extreme public anxiety related to these incidents, which may have been a major goal of the terror group responsible.

In addition to these actual infections, an even greater number of false threats of anthrax have been reported. In 1999, the FBI received an average of one false report per day of alleged anthrax use. False reports, both hoaxes and alarmed citizens misperceiving harmless material for anthrax, increased even more following the 2001 anthrax attack.

The only other successful use of a biological agent by a terror group in the United States occurred in 1984. In this event, 751 people were stricken with diarrhea resulting from the intentional contamination with Salmonella of a salad bar in Oregon. The bacteria were introduced by a religious cult trying to influence the results of a local election. No one died.

Defense against bioterrorism involves several factors: intelligence to disrupt the terrorists before they can use the weapons; early detection; availability of protective antibiotics; and immunization of selected populations (such as the military).

Infection From Medical Devices

Usually, people think of infection as occurring when microorganisms invade the body and adhere to specific cells. But microorganisms can also adhere to medical devices placed in the body—such as catheters, artificial joints, and artificial heart valves—and begin to grow. The microorganisms may be present on the device when inserted if the device was accidentally contaminated. Or infecting organisms from another site may spread through the bloodstream and lodge on an already implanted device. Because implanted material has no natural defenses, it is easy for the microorganisms to grow and spread, causing illness.

Last full review/revision February 2003

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