CHAPTER 14

Infection

Every tissue and organ of the human body can become infected with different living organisms. Infections are one of the leading causes for doctor visits and hospitalizations. The good news is that most infections are treatable and curable if caught early.

The Body's Response to Infection:

1. Temperature: Infections cause significant body changes. One such change is an elevation in the body temperature. This has two effects: it aids the body in fighting the infection and causes the person with a fever to feel ill. High fevers can cause weakness, muscle aches, headaches, and other unpleasant symptoms.
   
   Fever is useful in that it is an indicator that an infection may be present. There are, of course, many causes of elevated body temperature, but infection is by far the most common.
2. Lymph nodes: As blood flows out of the arteries to the capillaries which supply blood to individual tissues, most of it returns via the veins to the heart. Some proteins and cells, however, escape from the blood to bathe and nourish tissues. This fluid (lymph) does not re-enter the blood system directly, but instead migrates and flows through filters (the lymphatic system) before finally rejoining the circulatory systems at veins near the heart. The filters this fluid goes through are called lymph nodes. Lymph nodes are found throughout the body, like in the groin, armpits, neck, and behind the ears.
   
   When a tissue is infected, signals are sent to the blood vessels which release more proteins and cells to bathe the tissue to help fight infection. When a lymph node becomes activated by an infection, it swells to several times its original size and often becomes palpable. Such lymph nodes are often tender, and when palpable, may signify infection.
   
   There are, of course, other causes for enlarged lymph nodes, such as cancer. But again, infection is the most common cause of enlarged and tender lymph nodes.

3. Local signs of infection/inflammation: The body heals damaged or diseased tissue by causing an inflammatory response. Such responses bring cells and proteins to the damaged area to repair it. Inflammation is characterized by four signs and symptoms which are universally present wherever there is an infection:
   Pain: When a tissue or organ is infected, the chemicals and proteins released by the inflammatory reaction cause pain. This pain is caused in part by the swelling that occurs, and in part by irritation of nerve endings.
   
   Warmth: In the same way that the whole body can become feverish, the tissue at the site of infection becomes warm to touch.
   
   Swelling: When infection is present, since more cells, proteins, and fluids are coming to the area to repair the tissue, it swells.
   
   Redness: Blood flow to an infected area is increased when inflamed, so the area becomes reddened.

4. Bugs and drugs: There are four different classes of organisms which cause infection, and they are treated differently.
   
   Parasites: These are living organisms of different sizes which infect various body tissues and organs. They are unable to live alone outside the body since they derive their water and nutrition from the body. Many parasites have complicated life cycles in which they spend different stages of their life cycle inhabiting different animals. One specific malaria parasite, for example, spends part of its life cycle in the mosquito and the rest of its life cycle in the human bloodstream and liver.
   
   Parasitic infections are usually diagnosed by signs and symptoms, but the diagnosis is often assisted by seeing the parasites directly under the microscope. This would occur, for example, in samples of stool, blood, or urine. Millions and millions of people are infected with parasites, most of them live in areas of poor sanitation, which makes the spread of parasites more likely. Fortunately, there are many effective drugs which treat and often cure almost all parasitic infections.
   Bacteria: Bacteria are tiny unicellular organisms found throughout nature which can only be seen with a microscope. They live in the soil and in and on all plants and animals. Some are useful to the human body -- such as the bacteria which digest food in the intestines. Others live in and on the body without causing harm. Some, however, invade the tissues and cause dangerous and even life threatening infections. Bacteria are treated with antibiotics. However, certain antibiotics only kill certain bacteria.
   
   When the infection is simple, the doctor can assume which bacteria are likely to be causing the problem and treat with an appropriate antibiotic. When an infection is unusual, serious, or life threatening, doctors like to know which bacteria are causing the infection and which antibiotics will kill them effectively. To find this out a small culture swab is placed in contact with the area of infection (whether that be the back of the throat, or in a deep wound) and sent to the laboratory. There it is placed in a specially designed material in which the bacteria will grow. Several days later colonies of bacteria which have grown can be tested chemically and identified under the microscope. This helps the doctor choose the appropriate antibiotic. Additional tests can be done to make sure that the antibiotic will be effective, since some bacteria develop resistance to antibiotics and are no longer destroyed by their use.
   
   Viruses: Viruses are even smaller than bacteria. They have some, but not all the characteristics of living organisms. They reproduce, for example, but they do not grow or eat. Viruses reproduce by entering cells, hijacking them, and using their cellular processes to multiply. As a result the infected cells often die. There arc thousands of varieties of viruses, and they cause many types of diseases. For example, the common cold, flu, warts, chicken pox, mumps, measles, and many gastrointestinal diseases are caused by viruses. Most viruses do not respond well to antiviral medicines -- and most don't need to be treated since they run their course and the patient recovers. A few anti-virus medicines are effective for serious viral infections.
   
   Fungi: These are organisms with a great variety of structures and form. Some, like bread mold are very small, while others like mushrooms are very large. Some types of fungi infect the human body, but most do not. Common human fungal infections include: athlete's foot, ringworm, some diaper rashes, and vaginal yeast infections in women.

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