Infectious agents: From bacteria to worms

Infectious agents come in many shapes and sizes. Categories include:

• Bacteria
• Viruses
• Fungi
• Protozoans
• Helminths

Bacteria

Bacteria are one-celled organisms that can be seen 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.

Not all bacteria are harmful, and some bacteria that live in your body are helpful. For instance, Lactobacillus acidophilus — a harmless bacterium that resides in your intestines — helps you digest food, destroys some disease-causing organisms and provides nutrients.

Many disease-causing bacteria produce toxins — powerful chemicals that damage cells and make you ill. Other bacteria can directly invade and damage tissues. Some infections caused by bacteria include:

• Strep throat
• Tuberculosis
• Urinary tract infections

Viruses

Viruses are much smaller than cells. In fact, viruses are basically just capsules that contain genetic material. To reproduce, viruses invade cells in your body, hijacking the machinery that makes cells work. Host cells are often eventually destroyed during this process.

Viruses are responsible for causing many diseases, including:

• AIDS
• Common cold
• Ebola virus
• Genital herpes
• Influenza
• Measles
• Chickenpox and shingles

Antibiotics designed for bacteria have no effect on viruses.

Fungi

There are many varieties of fungi, and we eat several of them. Mushrooms are fungi, as are the molds that form the blue or green veins in some types of cheese. And yeast, another type of fungus, is a necessary ingredient in most types of bread.

Other fungi 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 and in people taking antibiotics or who have an impaired immune system. Fungi are also responsible for skin conditions such as athlete's foot and ringworm.

Protozoans

Protozoans are single-celled organisms that behave like tiny animals — hunting and gathering other microbes for food. Many protozoans call your intestinal tract home and are harmless. Others cause diseases, such as:

• Giardia
• Malaria
• Toxoplasmosis

Protozoans often spend part of their life cycles outside of humans or other hosts, living in food, soil, water or insects. Some protozoans invade your body through the food you eat or the water you drink. Others, such as malaria, are spread by mosquitoes.

Helminths

Helminths are among the larger parasites. The word "helminth" comes from the Greek word for worm. If these parasites — or their eggs — enter your body, they take up residence in your intestinal tract, lungs, liver, skin or brain, where they live off your body's nutrients. Helminths include tapeworms and roundworms.