An infection is a condition in which viruses, bacteria, fungi, or parasites enter the body and cause a state of disease. Such invaders are called pathogens. They damage cells of the body by adhering to and damaging the cell walls, releasing toxic substances or causing allergic reactions. The body has a set series of responses to infection, which mostly involve body chemicals, body tissues, and the immune system. It was recently reported that infection is the fourth leading cause of death in the United States and kills more people than cancer and heart disease combined.
Pathogens are everywhere in a person's daily environment: They may enter the body through breathing, ingested food or water, sexual contact, open wounds, or contact with contaminated objects. Having entered the body, pathogens begin to reproduce. Most pathogens are kept in check before they have a chance to multiply. If, however, the body is unable to keep the pathogens in balance, serious disease and even death may occur. Chronic infections may develop if the body has only limited control over a given pathogen. In that case, the infection will have a tendency to flare up in response to stress and weakness. Sepsis is a serious condition in which pathogens spread and circulate throughout the body via the bloodstream. This type of infection affects the entire body.
The body has many natural barriers to infection. For example, the harmless bacteria normally found on the skin, known as commensals, inhibit the growth of many pathogens. Sweat and oil gland secretions also protect the skin; and the skin itself offers a significant physical barrier that no bacteria are able to penetrate. Damage to the skin from burns, insect bites, surgery, or injuries may leave the skin open to infection. In addition to the physical barrier of the skin, many of the body's secretions such as tears, sweat, urine, and saliva contain chemicals that destroy pathogens.
The mucous membranes that line the passageways of the body secrete mucus, which contains enzymes and chemicals that kill or disable pathogens. The pathogens are then trapped in the mucus and filtered out or swallowed. Commensal bacteria also live on the mucous membranes; there they inhibit the spread and multiplication of pathogens just as they do on the skin. The digestive tract contains stomach acid, pancreatic enzymes, and other secretions that protect against infection. Peristalsis and the shedding of the lining of the intestinal tract also help to remove pathogens. The acid pH of the stomach and vagina is protective, as well as the length of the urethra in males. The flushing action of urine and feces as they are excreted also protect against infection.
A fever is defined as the elevation of the body temperature to at least 100°F (37.8°C). Fevers are a helpful part of the body's response to an infection, since most pathogens do not thrive at higher temperatures while the immune system's white blood cells (WBCs) work best in a warm environment. If the fever reaches levels of 102°F (38.9°C) or higher, it may have to be brought down to avoid seizures, dehydration, and tissue damage.
The second level of the body's defenses is the immune system. The white blood cells are a major part of this system. In response to the invasion of pathogens, WBCs are released from the bone marrow into the bloodstream. The main function of these WBCs, depending on their type, is to engulf pathogens and render them harmless, detoxify poisons, produce and release antibodies and chemicals, and clean up wastes left by other WBCs. The spleen, thymus, lymph system, and liver all have roles in the immune response. Successful removal of pathogens from the body often gives immunity against infection by that pathogen in the future.
Pathogens can be persistent. They may secrete enzymes, destroying tissues in the body to spread the infection more quickly and effectively. They may secrete chemicals that counteract the actions of the WBCs. Some pathogens release toxins that kill the surrounding cells. Many also have methods to evade being engulfed and destroyed. In addition, the body's own commensal bacteria may become pathogenic if something upsets their balanced state in the body. This loss of balance may often result from chronic illness, low stomach acid, recurrent use of antibiotics, cross-contamination through medical or sexual exploration, or a compromised immune system.
Patience Paradox, Teresa G. Odle, The Gale Group Inc., Gale, Detroit,