Infectious agents come in many shapes and sizes. They include:
- Bacteria.
- Viruses.
- Fungi.
- Protozoans.
- Helminths.
Bacteria
Bacteria are one-celled life forms called organisms that can be seen only with a microscope.
Not all bacteria are harmful. Some bacteria that live in the body are helpful. For instance, some bacteria that live in the intestines, help digest food, destroy disease-causing organisms and provide nutrients.
But bacteria may also cause illness. Many disease-causing bacteria make powerful chemicals called toxins that damage cells and make you ill. Other bacteria can get into 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 just capsules that hold genetic material. To reproduce, viruses invade cells in the body. They take over the process that makes cells work. In time, host cells often are destroyed during this process.
Viruses are responsible for causing many diseases, including:
- Common cold.
- Influenza.
- Measles.
- Chickenpox and shingles.
- Coronavirus disease 2019, also called COVID-19.
Antibiotics kill or block activities bacteria need to live or grow, but antibiotics don't work on viruses. Medicine that treats viral infections is called an antiviral. These medicines usually stop a virus from making copies of itself. They also may stop a virus from going into or leaving a cell.
Fungi
There are many types of fungi. People eat some of them. Mushrooms are fungi. So are the molds that form the blue or green veins in some types of cheese. And yeast is a type of fungus needed to make most breads.
Other fungi can cause illness. One example is the yeast candida. Candida can cause an infection of the mouth and throat called thrush. Thrush happens in infants and in people taking antibiotics or who have weakened immune systems. Fungi also cause skin conditions such as athlete's foot and ringworm.
Protozoans
Protozoans are single-celled life forms that act like tiny animals. They hunt and gather other microbes for food. Many harmless protozoans live in the intestinal tract. Others cause diseases, such as:
- Giardiasis.
- Malaria.
- Toxoplasmosis.
Protozoans often spend part of their life cycles outside of humans or other hosts. They live in food, soil, water or insects. Some protozoans enter the body through food or water. Others, such as the malaria protozoans, enter the body through mosquito bites.
Helminths
Helminths are among the larger parasites. The word helminth comes from the Greek word for worm. If these parasites or their eggs enter the body, they settle in the intestinal tract, lungs, liver, skin or brain. There, they live off the body's nutrients. Helminths include tapeworms and roundworms.