The human body is at constant war against disease-causing pathogens, or germs, which can enter the body through inhaled air, ingested food and water, broken skin, insect or animal bites, and sexual contact. Once on or inside the body, pathogens try to survive and reproduce. Most do not succeed. Many succumb to the body's internal heat or chemical environment, or to "friendly" bacteria; others are expelled in mucus, urine, sweat, and faeces. Those that survive do so by preying on healthy cells and tissue. Some pathogens can live only in certain types of tissue; others can spread through the body. The three-part process of invasion, growth, and the body's reaction constitutes infection, and though most infections are minor and short-lived, some evolve into serious - even life-threatening - diseases.

Once a pathogen invades, the body's immune system counterattacks. Proteins called antibodies team up with special white blood cells to neutralize and destroy pathogens. The immune system also is able to "remember" contacts with a particular pathogen so it can suppress repeat invasions. This cellular memory - sometimes aided by vaccines - gives the body immunity against countless disease - causing pathogens.

Rapid immune system response stops or weakens most - but not all - infections. Sometimes immune cells fail to recognize and attack pathogens, especially unfamiliar ones. At other times, the body's counterattack is not enough. People whose immune systems are weakened by fatigue, poor nutrition, certain medical treatments, or illnesses such as AIDS are more susceptible to infection than are people in good general health.

The pathogens that cause infections are bacteria, fungi, animal parasites, and viruses. Bacteria, fungi, and parasites generally invade body tissue, steal nutrients from healthy cells, and release toxins; some parasites which can range in size from single-celled protozoa to visible worms-actually kill healthy cells. Viruses are really sub-life forms that survive only by invading living cells and multiplying inside them.

Understandably, the parts of the body that are most accessible to pathogens are those most vulnerable to infection. Pathogens readily infect the eyes, ears, mouth, genitals, and the skin itself. Skin infections are extremely common and can be caused by any type of pathogen. Ear infections are usually bacterial; eye infections may be viral or bacterial. Other pathogens can enter the body through broken skin and infect the bloodstream.

Any type of pathogen can invade the gastrointestinal tract, usually through contaminated food or water. Most cases of food poisoning are caused by bacteria, while giardiasis, an intestinal affliction dreaded by hikers, results from drinking water contaminated by parasitic protozoa.

Most respiratory infections, including common colds, flu, and mild forms of pneumonia, are caused by inhaling or ingesting viruses. Severe cases of pneumonia are usually bacterial, as are whooping cough and strep throat. Many lesser-known respiratory ailments are fungal infections.

The urinary tract usually flushes out pathogens but is prone to infection when it is swollen, irritated, or obstructed. Ailments ranging from common bladder infections to serious kidney infections are usually bacterial.

Although many infections are confined to a body part or system, some infectious diseases spread through the body. Malaria, for example, is caused by a mosquito-borne parasite; tuberculosis is bacterial. AIDS, the deadliest infectious disease of recent times, is a viral disorder.

Diagnostic and Test Procedures
Many common infections are recognisable from their symptoms, but to treat infections that persist or whose potential severity is unknown, a doctor needs to determine the specific cause. Diagnosis usually is accomplished by lab analysis of blood, urine, faeces, or infected tissue samples.

Minor infections are of short duration and usually clear up on their own, but serious infectious diseases require medical treatment.

Conventional Medicine
Bacterial infections are cured by killing the bacteria with antibiotics, such as penicillin and its derivatives. Many fungal and parasitic infections also respond to antibiotics or antimicrobial drugs. The only downside is that overuse or misuse of antibiotics can suppress the body's own natural immune reactions.

Antibiotics are useless against viral infections. Although some antiviral drugs can reduce symptoms, it is difficult or impossible to develop more effective treatment because viruses mutate, or change forms, so often. Standard therapy for most viral infections is simply to relieve symptoms, prevent secondary bacterial infection, and allow the body to heal itself.

Over-the-counter and prescription medications can relieve typical symptoms and speed recovery from minor infections. Preventive vaccines are by far the best medicine against viral infection, and they are available for many infectious diseases.

Alternative Choices
Proper nutrition, rest, and stress-reducing activities can help prevent and combat infection. Various herbal, Chinese, and other alternative therapies can relieve symptoms and promote healing. See the entries for specific ailments for more information.

Be sure you and your children are vaccinated against infectious diseases, especially before starting school or traveling abroad.

To prevent the start of infection, refrigerate meat, seafood, and dairy products; wash raw vegetables and fruit thoroughly; keep a clean kitchen; wash your hands before and after handling food.

Practice good personal hygiene: Bathe regularly; wash your hands after using the toilet; sterilize cuts and wounds with anti-septic soap.

Additional Resources:

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Why does bacteria, fungus, and parasites respond to antibiotics? That's the question i have.

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