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.
E. coli O157:H7 is a bacterium responsible for food-borne infections linked to eating undercooked ground beef.
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.
The influenza virus takes over healthy cells, spreads through your body and causes illness. Signs and symptoms include fever, chills, muscle aches and malaise.
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.
Infection with candida fungus can lead to problems such as diaper rash, vaginal yeast infections and oral thrush.
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.
Cryptosporidium is a protozoan protected by a strong outer shell that allows it to survive outside the body for long periods of time.
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.
Infection by one type of roundworm, known as a hookworm, can cause problems in your small intestine or lungs. The average hookworm is about half an inch long.
Understanding infection vs. disease
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.
When to seek medical care
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
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
Blurred vision or other difficulty seeing
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.