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.
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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.
Symptoms and Diagnosis
Where an abscess is located and whether it interferes with the function of an organ or a nerve determine its symptoms. Symptoms can include pain, tenderness, heat, swelling, redness, and possibly a fever. An abscess that forms just under the skin usually appears as a visible bump. When an abscess is about to rupture, it develops a white center as the overlying skin grows thinner. An abscess deep inside the body often grows quite large before it causes symptoms. Unnoticed, a deep abscess is likely to spread infection throughout the body.
Doctors can easily recognize an abscess on or just beneath the skin but often miss a deep abscess. When a person has such an abscess, blood tests often reveal an abnormally large number of white blood cells. X-rays, ultrasound scanning, computed tomography (CT), or magnetic resonance imaging (MRI) can be used to determine the size and position of an abscess. Because abscesses and tumors often cause the same symptoms and produce similar images, a definitive diagnosis sometimes requires that a doctor obtain a sample of the pus or surgically remove the abscess for examination under a microscope.
Treatment
Often an abscess heals without treatment by rupturing and discharging its contents. Occasionally, the abscess disappears slowly without rupturing as the body destroys the ingfection and absorbs the debris. The abscess may leave a hard lump.
An abscess can be pierced and drained to relieve pain and promote healing. To drain a large abcess, a doctor must probe it to break down the walls and realese all the pus. When drained, large abscesses leave a large empty space (dead space), which a doctor may temporarily pack with gauze. Sometimes, inserting temporary artificial drains (usually thin plastic tubes) is necessary.
Because an abscess doesn't have a blood supply, antibiotics usually aren't helpful. After an abscess has been drained, antibiotics may be prescribed to prevent a recurrence. Antibiotics also are taken when an abscess spreads infection to other parts of the body. A laboratory analysis of bacteria from the pus helps a doctor select the most effective antibiotic.
Notes:
bump припухлость
promote способствовать
select подбирать
Text D Abdominal abscesses
Abscesses may form below the diaphragm, in the middle of the abdomen, in the pelvis, or behind the abdominal cavity. Abscesses also may form in or around any abdominal organ, such as the kidneys, spleen, pancreas, or liver, or in the prostate gland. Often, abdominal abscesses are caused by injury, infection or perforation of the intestine, or infection of another abdominal organ.
An abscess below the diaphragm may form when infected fluid, for example, from a ruptured appendix, is moved upward by the pressure of abdominal organs and by the suction created when the diaphragm moves during breathing. Symptoms may include a cough, painful breathing, and pain in one shoulder—an example of referred pain that occurs because the shoulder and the diaphragm share the same nerves and the brain incorrectlv interprets the source of the pain.
Abscess in the midabdomen may result from a ruptured appendix, a perforated large intestine, inflammatory bowel disease, or diverticular disease. The abdomen is usually painful in the area of the abscess.
Pelvic abscesses result from the same disorders that cause abscesses in the midabdomen and from gynecologic infections. Symptoms may include abdominal pain, diarrhea from intestinal irritation, and an urgent or frequent need to urinate caused by bladder irritation.
Abscesses behind the abdominal cavity lie behind the peritoneum, the membrane that lines the abdominal cavity and organs. The causes, which are similar to those of other abscesses in the abdomen, include information of the appendix and of the pancreas. Pain usually in the lower back, worsens when the person moves the leg at the hip.
Abscesses in the kidneys are caused either by bacteria from an infection traveling to the kidneys through the bloodstream or by a urinary tract infection traveling to the kidney and then spreading to the kidney tissue. Abscesses on the surface of the kidneys (perinephric abscesses) are nearly always caused by the rupture of an abscess inside the kidney, which spreads the infection to the surface and the surrounding tissue. Symptoms of a kidney abscess include a fever, chills, and pain in the lower back. Urination may be painful, and sometimes the urine is bloody.
Abscesses in the spleen are caused by an infection traveling through the bloodstream to the spleen, by an injury to the spleen, or by the spread of an infection from a nearby abscess, such as one below the diaphragm. Pain may occur in the left side of the abdomen, the back, or the left shoulder.
Abscesses inside the pancreas typically form after an attack of acute pancreatitis. Symptoms such as fever, abdominal pain, nausea, and vomiting often begin a week or more after a person recovers from pancreatitis.
Liver abscesses may be caused by bacteria or by amebas (single-celled parasites). Amebas from an intestinal infection reach the liver through the lymphatic vessels. Bacteria can reach the liver through the lymphatic vessels. Bacteria can reach the liver from an infected gallbladder; a penetrating or blunt wound; an infection in the abdomen, such as a nearby abscess; or an infection carried by the bloodstream from elsewhere in the body. Symptoms of liver abscessesinclude loss of appetite, nausea, and a fever. A person may or may not have abdominal pain.
Prostate abscesses usually result from a urinary tract infection that leads to a prostate gland infection. These abscesses most commonly occur in men between ages 40 and 60. Typically, a man with a prostate abscess experiences painful, frequent, or difficult urination. Less commonly, he feels internal pain at the base of the penis and notices pus or blood in his urine.
Diagnosis and Treatment
In nearly all cases of abdominal abscesses, the pus must be drained, either by surgery or by a needle inserted through the skin. To guide the placement of the needle, a doctor uses computed tomography (CT) or ultrasound scanning. Laboratory analysis of the pus identifies the infecting; organism so that the most effective antibiotic can be selected.
Notes:
diverticular disease дивертикулез
disintegrate разделять на части
placement место расположения
impair ухудшать
absorb the debris всасывать инородное тело