Text C Abscesses

SECTION 16

INFECTIOUS DISEASES

Infections

Microorganisms are tiny living creatures, such as bacteria and viruses. Microorganisms are present everywhere. Despite their overwhelming abundance, relatively few of the thousands of species of microorganisms invade, multiply, and produce illness in people.

Many microorganisms live on the skin and in the mouth, upper airways, intestine, and genitals without causing disease. Whether a microorganism lives as a harmless companion to a person or invades and causes disease depends on the nature of the microorganism and on the state of the person's natural defenses.

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.

Several steps help protect people against infection. Hand washing is an effective way of preventing the transmission of infectious microorganisms from one person to another. Hand washing is particularly important for people who handle food or who have frequent physical contact with other people. People visiting hospital patients who are seriously ill may be asked to wash their hands and put on a gown, mask, and gloves before entering the person's room.

Sometimes, to prevent an infection, antibiotics are given to people who do not yet have an infection. This preventive measure is called prophylaxis. Antibiotics are given prophylactically before dental procedures to people with abnormal heart valves. Many healthy people who undergo certain types of surgery—particularly abdominal surgery and organ transplantation—also require prophylactic antibiotics.

Vaccination also can prevent infections. People who are at increased risk of developing infections (especially infants, children, older people, and people with AIDS) should receive all the vaccinations necessary to reduce this risk.

Notes:

clostridium tetani палочка столбняка

multiply размножаться

virulence заражение

resist препятствовать, не поддаваться

cryptococcus дрожжеподобные грибы

break down разрушать

require требовать

Text A Fever

Body temperature increases (fever) as a protective response to infection and injury. The elevated body temperature enhances the body's defense mechanisms, although it can cause discomfort for the person. Temperature is considered elevated when it is higher than 37,7 C as measured by an oral thermometer. Although 36,6C is considered "normal" temperature, body temperature varies throughout the day, being lowest in the early morning and highest in the late afternoon—sometimes reaching 37,7° C.

A part of the brain called the hypothalamus controls body temperature. Fever results from an actual resetting of the hypothalamus's thermostat. The body raises its temperature to a higher level by moving (shunting) blood from the skin surface to the interior of the body, thus reducing heat loss. Shivering (chills) may occur to increase heat production through muscle contraction. The body's efforts to conserve and produce heat continue until blood reaches the hypothalamus at the new, higher temperature. The new, higher temperature is then maintained. Later, when the thermostat is reset to its normal level, the body eliminates excess heat through sweating and shunting of blood to the skin.

Fever may follow a pattern: sometimes temperature peaks every day and then returns to normal. Alternatively, fever may be remittent, in which the temperature varies but does not return to normal. Certain people (for example, alcoholics, the very old, and the very young) may experience a drop in temperature as a response to severe infection.

Substances that cause fever are called pyrogens. Pyrogens can come from inside or outside the body. Microorganisms and the substances they produce (such as toxins) are examples of pyrogens formed outside the body. Pyrogens formed inside the body are usually produced by monocytes. Pyrogens from outside the body cause fever by stimulating the body to release its own pyrogens. However, infection is not the sole cause of fever; fever also may result from inflammation, cancer, or an allergic reaction.

Usually, fever has an obvious cause, which is often—but not always—an infection (such as influenza, pneumonia, a urinary tract infection, or some other infection) that a doctor can easily diagnose with a brief history, physical examination, and occasionally a few simple tests, such as a chest x-ray and urine tests. Sometimes, however, the cause is not readily discernible.

If fever continues for several days and has no obvious cause, a more detailed investigation is required. There are many potential causes of such a fever; common causes in adults include infections, diseases caused by antibodies against the person's own tissues (autoimmune diseases), and an undetected cancer (especially leukemia or lymphoma).

To determine the cause of a fever, a doctor begins by asking a person about present and previous symptoms and diseases, drugs currently being taken, exposure to infections, and recent travel. The pattern of the fever usually does not help with the diagnosis. There are, however, some exceptions: a fever that recurs every other day or every third day is typical of malaria.

Recent travel (especially overseas) may give clues to the cause of a fever, because some infections occur only in certain areas. For example, coccidioidomycosis (a fungal infection) occurs almost exclusively in the southwestern United States. A history of exposure to certain materials or animals also is important. For example, a person who works in a meatpacking plant is more likely to develop brucellosis.

After asking questions, the doctor performs a thorough physical examination to find a source of infection or evidence of disease. Blood and other body fluids may be sent to the laboratory to try to grow the microorganism in a culture. Other blood tests can be used to detect antibodies against specific microorganisms. Increases in the white blood cell count usually indicate infection. The differential count (the proportion of different types of white blood cells) gives further clues. An increase in neutrophils, for example, suggests an acute bacterial infection. An increase in eosinophils suggests the presence of parasites—for example, tapeworms or roundworms.

When a person has a fever of at least 38,3° C for several weeks and extensive investigation fails to reveal a cause, a doctor may refer to it as a fever of unknown origin. In such cases, the cause may be an unusual chronic infection or something other than infection, such as a connective tissue disease, cancer, or some other disease. Ultrasonography, computed tomography (CT), or magnetic resonance imaging (MRI) may help a doctor diagnose the cause. Injection of white blood cells labeled with a radioactive marker can be used to identify areas of infection or inflammation. If these test results are negative, the doctor may need to obtain a biopsy specimen from the liver, bone marrow, or another suspected site. The specimen is then examined under a microscope and cultured.

Because fever helps the body defend against infection, there is some debate as to whether it should be routinely treated. However, a person with a high fever generally feels much better when the fever is treated.

Drugs used to lower body temperature are called antipyretics. The most effective and widely used antipyretics are acetaminophen and the nonsteroidal anti-inflammatory drugs (NSAIDs), such as aspirin and ibuprofen.However,aspirin should not be given to children and teenagers to treat a fever because it increases the risk of Reye's syndrome, which can be fatal.

Notes:

remittent перемежающий

elevate повышать

drop in temperature снижение температуры

reset вновь установить

pyrogen пироген

shivering озноб

identify определить, найти

Text B Inflammation

Any injury, including an invasion by microorganisms, causes a complex reaction called inflammation in the affected area. Inflammation occurs as a result of many different conditions. Through the release of different substances from the damaged tissue, inflammation directs the body's defenses to wall off the area, attack and kill any invaders, dispose of dead and damaged tissue, and begin the process of repair. However, inflammation may not be able to overcome large numbers of microorganisms.

During inflammation, the blood supply increases. An infected area near the surface of the body becomes red and warm. The walls of blood vessels become more porous, allowing fluid and white blood cells to pass into the affected tissue. The increase in fluid causes the inflamed tissue to swell. The white blood cells attack the invading microorganisms and release substances that continue the process of inflammation. Other substances trigger clotting in the tiny vessels (capillaries) in the inflamed area, which delays the spread of the infecting microorganisms and their toxins. Many of the substances produced during inflammation stimulate the nerves, producing pain. Reactions to the substances released during inflammation include the chills, fever, and muscle aches that commonly accompany infection.

Notes:

wall off отделять

pass into проникать в

tiny vessels маленькие сосуды

Text C Abscesses

An abscess is an accumulation of pus, usually caused by a bacterial infection.

When bacteria invade healthy tissue, the infection spreads through the area. Some cells die and disintegrate, leaving spaces where fluid and infected cells accumulate. White blood cells, the body's defenders against infection, move into these spaces, and after engulfing bacteria, they die. The dead white blood cells accumulate as pus, a creamy substance that fills the area. As pus collects, healthy tissue is pushed aside. Tissue eventually grows around the abscess and walls it off; this is the body's attempt to prevent the further spread of infection. If an abscess ruptures internally, the infection may spread either inside the body or under the skin surface, depending on the location of the abscess.

A bacterial infection may lead to an abscess in several ways. For example, a puncture wound from a dirty needle may force bacteria under the skin. Or bacteria may spread from an infection elsewhere in the body. Also, bacteria that normally live on the body but cause no harm can somemetimes cause an abscess. The chances of an abscess forming increase if there is dirt or a foreign body in the infected area; if the area of bacteria invasion has a poor blood supply, as occurs in diabetes; or if the person's immune system is impaired, as occurs in AIDS.

Abscesses can develop anywhere in the body, including the lungs. A mouth, rectum, and muscles. They are fairly common in or just below

in the skin, especially on the face.

Дата добавления: 2015-11-16