What Is a Virus?

Contributor: Felicia Sabur. Lesson ID: 11642

You're familiar with the concept of computer viruses. Did you know people got viruses before there were computers? Seriously, viruses are destructive, and you will research them and related diseases.


Life Science

learning style
personality style
Grade Level
High School (9-12)
Lesson Type
Quick Query

Lesson Plan - Get It!


They are teeny tiny invaders that can't be seen with the naked eye. However, if you have ever had the flu, you know they can have a major impact on your health.

  • Can you name three viruses that made headline news in recent years?

HIV, Zika, and Ebola are three viruses that were in the news in recent years (2017).

Viruses are the smallest type of microbes. Scientists argue over whether they are complex molecules or simple organisms. A virus is not considered living because it does not have all of the characteristics of life. Viruses do not respire, grow, or reproduce on their own.*

If you haven't already, print Viruses and Bacteria Unit Workbook from Downloadable Resources in the right-hand sidebar and fill out the Lesson Two Vocabulary section and the What Is a Virus? section as you go through this lesson. If you did not complete the previous Related Lesson in our Viruses and Bacteria series, please go to the right-hand sidebar.

A virus is a type of microorganism that cannot reproduce or grow without the help of a living host cell. A host cell is a cell that a virus uses to inject its DNA into, in order to make copies of the virus. Viruses can even infect bacteria. The type of virus that infects bacteria is called a bacteriophage.

The figure below, from Rice University, shows just how small viruses really are:

relative sizes of microbes on a lagarithmic scale

Image by OpenStax is licensed under the CC BY 4.0 license.

The shape of a virus determines what types of cell it can infect. The shape of a virus is determined by the proteins in its coat. Viruses come in four main shapes:

  • Helical
  • Polyhedral (also called icosahedral)
  • Enveloped (also called spherical)
  • Bacteriophage (also called complex)

The Structure of a Virus

The capsid is the protein shell that protects the genetic material of a virus, and the capsomere is a subunit of the capsid. All viruses contain a nucleic acid core that contains either DNA or RNA. The nucleic acid core only contains the genetic materials and coded instructions needed to make exact copies of the virus.

After viewing Viral Structure by MesquiteScience, finish all questions in the What is a Virus? section of your unit workbook:


If you would like a more in-depth look at the subject, watch Virus structure and classification | Cells | MCAT | Khan Academy, by khanacademymedicine:


So far, you have learned that a virus is the smallest type of microbe. Viruses are non-living and can only reproduce by invading a host cell from a living organism. There are four basic shapes of viruses and different parts of a virus.

Continue on to the Got It? section to create a model virus!

*For more information about the characteristics of a living organism, explore the Elephango lessons under Additional Resources in the right-hand sidebar.

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