Infectious Agents: Definition & Types

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  • 0:00 What Are Infectious Agents?
  • 0:26 Bacteria
  • 1:41 Virus
  • 2:40 Fungi
  • 3:24 Parasites
  • 4:27 Lesson Summary
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Lesson Transcript
Instructor: Katie Chamberlain

Katie has a PhD in Microbiology and has experience preparing online education content in Biology and Earth Science.

Infectious agents are tricky little invaders. They sneak in, use your body, and often make you sick in the process. In this lesson, you will learn about the different types as well as some common examples of each.

What Are Infectious Agents?

An infectious agent is something that infiltrates another living thing, like you. When an infectious agent hitches a ride, you have officially become an infected host. There are four main classes of infectious agents: bacteria, viruses, fungi, and parasites. This fab four can infect all sorts of living things.


Bacteria are single-celled organisms without a nucleus. It may seem like they are simple critters, but bacteria have tons of characteristics, and they are very diverse. They can vary in shape, size, structure, and even the environments they are built for. Because bacteria come in so many flavors, they are found in all sorts of places from the deepest underwater caves to the food in your lunchbox.

They are even on you and inside of you at all times. In fact, your intestine has a permanent infection. But, don't worry, these gut bacteria are actually very helpful for digestion. Many people take probiotic supplements to encourage the growth of this group of healthy bacteria.

If a pro-biotic is meant to be pro-bacteria, then what do you think an anti-biotic does? Yep, an antibiotic is harmful to bacteria. Doctors prescribe antibiotics to help your immune system out when you have a bacterial infection.

Bacteria can cause different symptoms depending on which bug is involved (bacteria are sometimes called bugs even though they are not insects). Some examples of bacterial infections are strep throat, tuberculosis, E. coli, anthrax, salmonella, syphilis, and bubonic plague.


Another type of infectious agent is a virus. They are even tinier than bacteria. A virus isn't technically alive because it cannot function unless it has a host. On their own, they are just little packages of a DNA or RNA genome inside a protein shell called a capsid. Their favorite and only activity is to hang around in an 'off' position and wait.

If they get inside a host, they will whir into action. Their only goal is to hijack the host cell's machinery and replicate themselves. This can occur right away or they might lay dormant for a bit before they replicate. After new viruses have been made, they go out into the world and the cycle starts over.

Like a bacterial infection, a viral infection can have different symptoms depending on which virus is doing the infecting. Some examples of viral infections are the common cold, the flu, rabies, ebola, herpes, HIV, HPV, and hepatitis.

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