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Bacteria, viruses and other infectious organisms live everywhere. You can find them in the air; on food, plants and animals; in the soil and in the water; and on just about every other surface — including your own body. They range in size from microscopic single-cell organisms to parasitic worms that can grow to several feet in length.

Most of these organisms (microbes) won't harm you. But others can cause infection. Your immune system protects you against an abundance of these infectious agents, and at times, it's a tough task. Viruses and bacteria are cunning adversaries, constantly seeking new ways to breach your immune system's defenses.

But you can give your immune system a fighting chance by understanding a little bit about the various kinds of microbes, what you can do to avoid infection and under what circumstances you should seek medical care.

Bacteria
Bacteria are one-celled organisms visible only with a microscope. They're so small that if you lined up a thousand of them end to end, they could fit across the end of a pencil eraser. They're shaped like short rods, spheres or spirals. They're usually self-sufficient and multiply by subdivision.

Among the earliest forms of life on earth, bacteria have evolved to thrive in a variety of environments. Some can withstand searing heat or frigid cold, and others can survive radiation levels that would be lethal to a human being. Many bacteria, however, prefer the mild environment of a healthy body.

Not all bacteria are harmful. In fact, less than 1 percent cause disease, and some bacteria that live in your body are actually good for you. For instance, Lactobacillus acidophilus — a harmless bacterium that resides in your intestines — helps you digest food, destroys some disease-causing organisms and provides nutrients to your body.

But when infectious bacteria enter your body, they can cause illness. They rapidly reproduce, and many produce toxins — powerful chemicals that damage specific cells in the tissue they've invaded. That's what makes you ill. The organism that causes gonorrhea (gonococcus) is an example of a bacterial invader. Others include some strains of the bacterium Escherichia coli — better known as E. coli — which cause severe gastrointestinal illness and are most often contracted via contaminated food. Other conditions caused by bacteria include strep throat and staph infection.

Viruses
In its simplest form, a virus is a capsule that contains genetic material — DNA or RNA. Viruses are even tinier than bacteria. To see them, scientists must use an electron microscope, a high-powered instrument that produces enlarged images of minute objects. To put their size into perspective, consider that, according to the American Society for Microbiology, if you were to enlarge an average virus to the size of a baseball, the average bacterium would be about the size of the pitcher's mound. And just one of your body's millions of cells would be the size of the entire ballpark.

The main mission of a virus is to reproduce. However, unlike bacteria, viruses aren't self-sufficient — they need a suitable host to reproduce. When a virus invades your body, it enters some of your cells and takes over, instructing these host cells to manufacture what it needs for reproduction. Host cells are eventually destroyed during this process. Polio, AIDS and the common cold are all viral illnesses.

Fungi
Molds, yeasts and mushrooms are types of fungi. For the most part, these single-celled organisms are slightly larger than bacteria, although some mushrooms are multicelled and plainly visible to the eye — for instance, the mushrooms you may see growing in a wooded area or even in your backyard. Mushrooms aren't infectious, but certain yeasts and molds can be.

Fungi live in the air, water, soil and on plants. They can live in your body, usually without causing illness. Some fungi have beneficial uses. For example, penicillin — an antibiotic that kills harmful bacteria in your body — is derived from fungi. Fungi are also essential in making certain foods, such as bread, cheese and yogurt.

Other fungi aren't as beneficial and can cause illness. One example is candida — a yeast that can cause infection. Candida can cause thrush — an infection of the mouth and throat — in infants, in people taking antibiotics and in people with impaired immune systems. It's responsible for most types of infection-induced diaper rash.

Protozoa
Protozoa are single-celled organisms that behave like tiny animals — hunting and gathering other microbes for food. Protozoa can live within your body as a parasite. Many protozoa inhabit your intestinal tract and are harmless. Others cause disease, such as the 1993 Cryptosporidium parvum invasion of the Milwaukee water supply, sickening more than 400,000 people. Often, these organisms spend part of their life cycle outside of humans or other hosts, living in food, soil, water or insects.

Most protozoa are microscopic, but there are some exceptions. One type of ocean-dwelling protozoa (foraminifer) can grow to more than 2 inches in diameter.

Some protozoa invade your body through the food you eat or the water you drink. Others can be transmitted through sexual contact. Still others are vector-borne, meaning they rely on another organism to transmit them from person to person. Malaria is an example of a disease caused by a vector-borne protozoan parasite. Mosquitoes are the vector transmitting the deadly parasite plasmodium, which causes the disease.

Helminths
Helminths are among the larger parasites. The word "helminth" comes from the Greek for "worm." If this parasite or its eggs enter your body, they take up residence in your intestinal tract, lungs, liver, skin or brain, where they live off the nutrients in your body. The most common helminths are tapeworms and roundworms.

The largest of the roundworms range in length from 6 to 14 inches. But imagine the largest of the tapeworms — they can grow to be 25 feet or longer. Tapeworms are made up of hundreds of segments, each of which is capable of breaking off and developing into a new tapeworm.

There's a distinct difference between infection and disease. Infection, often the first step, occurs when bacteria, viruses or other microbes enter your body and begin to multiply. Disease occurs when the cells in your body are damaged — as a result of the infection — and signs and symptoms of an illness appear.

In response to infection, your immune system springs into action. An army of white blood cells, antibodies and other mechanisms goes to work to rid your body of whatever's causing the infection. For instance, in fighting off the common cold, your body might react with fever, coughing and sneezing.

What's the best way to stay disease-free? Prevent infections from happening in the first place. You can prevent infection through simple tactics such as regular hand washing, vaccinations and appropriate medications.

• Hand washing. Often overlooked, hand washing is one of the easiest and most effective ways to protect yourself from most infections. Wash your hands thoroughly before preparing or eating food, after coughing or sneezing, after changing a diaper and after using the toilet. When soap and water aren't readily available, hand-sanitizing gels can offer protection.
• Vaccines. Vaccination is your best line of defense for certain diseases. As researchers understand more about what causes disease, the list of vaccine-preventable diseases continues to grow. Currently there are more than a dozen. Many vaccines are given in childhood, but adults still need to be routinely vaccinated to prevent some illnesses, such as tetanus and influenza.
• Medicines. Some medicines can help you from becoming susceptible to infectious agents. For example, taking an anti-parasitic medication might protect you from contracting malaria if you travel to or live in an area where your risk is high. After exposure to certain organisms — such as those that cause bacterial meningitis — your doctor may prescribe antibiotics to lower your risk of infection. Or you may choose an over-the-counter antibiotic cream or ointment for minor cuts and scrapes. But long-term, indiscriminate use of antibiotics isn't recommended in most cases. It won't prevent bacterial infections and instead may result in a more resistant, harder-to-treat strain of bacteria when infections do occur.

If you think you've contracted an infectious disease, contact your doctor. Although some infectious diseases, such as the common cold, might not require a visit to the doctor, others might call for the expertise of a trained professional.

Seek medical care if you suspect that you have an infection and you have experienced any of the following:

• An animal bite
• Difficulty breathing
• A cough lasting longer than a week
• A fever of 100.4 F (38.0 C) or more
• Periods of rapid heartbeat
• A rash, especially if it's accompanied by a fever
• Swelling
• Blurred vision or other difficulty seeing
• Vomiting
• An unusual or severe headache

Your doctor can perform diagnostic tests to find out if you're infected, the seriousness of the infection, and how best to treat that infection.