Disease-Causing Agents in the Environment: Definition and Types

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  • 0:08 Disease-causing Agents
  • 1:03 Viruses
  • 1:59 Bacteria
  • 3:14 Protozoa
  • 4:00 Parasitic Worms
  • 5:00 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

In this video lesson, you will identify different disease-causing agents. You will also explore examples of types of disease-causing agents and their effects on human health.

Disease-Causing Agents

Being sick is no fun, but it can actually be good for your body. When you're sick, your body has a chance to build up an immunity to whatever is causing you to feel badly.

This is because your immune system is like a small but powerful army - it attacks foreign objects and organisms that may harm you and then stores that information so it can remember how to attack again in the future.

But, how do we get sick in the first place? Diseases, which are any illness-causing conditions that affect an organism's body, are transmitted by disease-causing agents, which are biological pathogens that cause or spread disease. Disease-causing agents are delivered to your body through various carriers, and they can get inside in all sorts of ways. The disease is worrisome, but the disease-causing agent is just as problematic because it is what actually transports the disease that makes you sick.


Viruses are the disease-causing agent that most people are familiar with. They cause colds, the flu, HIV, meningitis, polio, chicken pox, rabies, West Nile, and a number of other diseases. Viruses themselves aren't living things, but they do get transmitted between living organisms.

Take West Nile, for example: this is a virus that is carried by mosquitoes. A mosquito bites you and spreads the West Nile virus into your blood. This unwelcome guest makes a home in your body, attacking your living cells and then reproducing.

Viruses can often be controlled by vaccines, which are exposures to an inert version of the virus or a safe agent that resembles the virus. This allows your body to build up that immune response before you ever get the virus, so that instead of getting sick first, you already have your army ready to fight ahead of time.


Bacteria get a bad rap. Many people confuse bacteria for viruses, and in fact, most of the bacteria that you come in contact with are either not harmful or actually provide some health benefits to you. You are literally covered with good bacteria, both inside and outside your body. Your digestive system wouldn't be able to function properly without the numerous types of bacteria that help break down and absorb the nutrients in all the foods you eat.

There are, of course, some bad bacteria and these can be quite dangerous. In contrast to viruses, bacteria are living organisms, but they are usually made up of just one cell. Bacteria are especially hard to control because they are extremely prolific - they can multiply into colonies of millions and billions of organisms in a very short amount of time.

Bacteria cause infections and disease. As mentioned before, meningitis is caused by a virus, but there is also a bacterium that will cause a different type of meningitis. Pneumonia, strep throat, Lyme disease, and many intestinal conditions are caused by bacteria. Antibiotics are used to treat diseases caused by bacteria because they are medications that kill bacterial infections.


Protozoa are also small, unicellular organisms. They can live in all different types of environments and are able to move around really well. They are parasitic, which means that they live and feed off of a host organism.

Amoebas and sleeping sickness are examples of diseases caused by protozoa, as well as some common gastrointestinal ailments. Amoebas are actually a type of protozoa, but we use the term to describe disease caused by them as well. Protozoa are sneaky little guys because they can slip into your body through food, water, or skin contact.

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