Infectious Agents: Viruses, Viroids & Prions

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  • 0:00 Non-Living Killers
  • 0:36 What Are Viruses?
  • 2:15 What Are Viroids
  • 3:03 What Are Prions?
  • 4:09 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
While we tend to think of things that can actively kill us as being living entities, the fact is that non-living entities can actively kill us, as well. This lesson explores some of those entities, including viruses, viroids, and prions.

Non-Living Killers

What's alive and can kill you? Let's see, there's bacteria, lions, people, and so much more.

What's not alive and can kill you? Perhaps a falling rock or a tsunami, as well as viruses, viroids, and prions.

Viruses, viroids, and prions are not considered to be living entities even though they, like bacteria, are considered to be infectious agents. The reasons why they aren't considered to be alive are beyond this lesson's scope and debate, but you will learn what viruses, viroids, and prions are and know a few famous examples of each.

What Are Viruses?

A virus is a very small infectious agent that doesn't have independent metabolic function and is unable to replicate outside of its host. What does this mean? Any entity that is said to be metabolically inert cannot create its own energy. If it cannot create energy, then it cannot grow or reproduce.

You, as a living entity, rely on your metabolism to provide you with energy to talk, run, and reproduce. Viruses have no such energy generating mechanism. Furthermore, a virus cannot reproduce unless it takes over a host cell.

There are many examples of viruses:

  • Rhinovirus, the most common cause of the common cold
  • Influenza virus, the one that causes the flu
  • HIV (human immunodeficiency virus), the one that leads to AIDS if left untreated

A virus, in the simplest structural sense, contains an outer protective covering encasing nucleic acids, DNA, or RNA. We can liken this simplistic viral structure to a hazelnut. The nut has a hard shell encasing the soft internal nut, with the nut itself being the nucleic acid in our metaphor.

This nucleic acid, DNA, or RNA, is the code that allows for the virus to take over a host cell and use it to reproduce. Think of these nucleic acids as, well, viruses on your computer. Computer viruses are pieces of computer code that take over your machine for whatever purpose. DNA and RNA are viral codes that take over a host cell for, namely, the purpose of reproduction.

What Are Viroids?

Now, imagine this virus, this hazelnut, as simply the soft nut without the external protective shell. This type of infective agent, an infectious particle composed solely of a piece of single-stranded circular RNA, is called a viroid.

Again, a viroid is an infectious agent composed exclusively of a piece of nucleic acid, a piece of single-stranded circular RNA. It's like a virus without its protein covering.

Viroids are believed to mainly affect plants, and they include:

  • Potato spindle tuber viroid
  • Coconut cadang-cadang viroid
  • Hop stunt viroid

The end result for plants affected by such viroids is everything from poor growth to death.

What Are Prions?

And I saved the wackiest non-living infectious agent that can kill you for last! It's called a prion, a transmissible and infectious protein responsible for neurodegenerative diseases. Neurodegenerative means that the nervous system undergoes a progressive loss of its normal structure and function. It doesn't happen overnight.

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