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Introduction to Viruses


Coronavirus (ktsimage, iStockphoto)

Learn about the basics of viruses in this backgrounder.

Viruses have a big impact on humans. Coronavirus 2019 (COVID-19), the common cold, the flu, Severe Acute Respiratory Syndrome (SARS) and diseases like Acquired Immunodeficiency Syndrome or AIDS are all caused by viral infections. Scientists have worked for hundreds of years to understand viruses and find better ways of fighting them.

Virus Structure

Viruses are very small. They are usually 17 to 400 nanometres in diameter. This is about 1000 times smaller than the diameter of a human hair. They are even 100 times smaller than most bacteria. Because viruses are so small, they have a simple cell structure. They are much simpler than animal or bacterial cells.

Viruses are made up of three basic parts:

General structure of a virus
General structure of a virus (©2020 Let’s Talk Science).
  1. Nucleic acid: A set of genetic material. This is either Deoxyribonucleic acid (DNA) or Ribonucleic acid (RNA). It is packaged in a protein shell.
  2. Capsid: A protein coat that surrounds the DNA or RNA to protect it. The capsid is made of proteins called capsomeres.
  3. Envelope: A covering for the capsid. This is made up of phospholipids and glycoproteins. Phospholipids form a fatty layer. The glycoproteins are a mix of proteins and carbohydrates (complex sugars). Not all viruses have envelopes. The ones that do not are called naked or non-enveloped.

Viruses can be simple or complex. They have many shapes. Like bacteria, they can be classified based on their shape. Pictures of viruses can look like something out of science fiction. Some have heads that are polyhedral, or many-­sided three-dimensional shapes. These are connected to little jointed shapes that look like legs. Other viruses look like round popcorn.

Viruses can be placed in four main categories:

Helical viruses­: These look like long rods. They can be rigid or flexible. The helical virus shown in the picture below is a tomato mosaic virus. This causes the leaves and fruit of tomato plants to appear blotchy.

Helical virus
Helical virus (Let’s Talk Science using an image by VectorMine via iStockphoto).

Polyhedral viruses: These are many-­sided viruses. Their capsids can have different numbers of sides. Most polyhedral viruses have 20 triangular sides and 12 vertices (corners). The polyhedral virus shown in the picture below is the adenovirus. This causes respiratory illnesses.

Polyhedral virus
Polyhedral virus (Let’s Talk Science using an image by VectorMine via iStockphoto).

Enveloped viruses: These are shaped like spheres because they have a protein, fat or carbohydrate coat over their capsid. Coronaviruses are a type of enveloped virus. 

The coronavirus disease outbreak of 2019 is caused by a virus like this. So is influenza. Severe Acute Respiratory Syndrome (SARS) and Middle East Respiratory Syndrome (MERS) are also caused by enveloped viruses. The enveloped virus shown in the picture below is an influenza virus. This causes respiratory illnesses like coughs and sore throats.

Enveloped virus
Enveloped virus (Let’s Talk Science using an image by VectorMine via iStockphoto).

Complex viruses: These have complicated structures. Capsides can be attached to structures that look like legs called tail fibres.The complex virus shown in the picture below is a bacteriophage. This virus infects bacteria.

Bacteriophage (Let’s Talk Science using an image by VectorMine via iStockphoto).

Viral Life Cycle

No matter what they look like, viruses are not actually alive. Viruses do not have a metabolism. A metabolism is all of the chemical reactions that a living cell or organism needs to live. Because viruses do not have metabolism, we do not consider them to be alive. 

Viruses also cannot reproduce on their own. A virus must have a host cell. This could be a bacteria, plant or animal cell. The virus needs to live inside the host cell to make more viruses. Here are the steps of the viral life cycle.

Viral Life Cycle
Viral Life Cycle (©2020 Let's Talk Science).
Infographic - Text version

1.Attachment: This is sometimes called absorption. This is when the virus attaches to the host cell wall.
2.Penetration: This is when the nucleic acid or genetic information of the virus moves through the cell membrane into the host cell.
3.Replication: This is also called biosynthesis. Once inside the host cell, the virus forces the host cell to make the components it needs for its replication.
4.Assembly: This is also called maturation. This is when the newly produced virus components are assembled into new viruses.
5.Release: This is when the completed viruses are released from the cell. They can now infect other cells and repeat the whole process again.


This is the process that happens in your body when you have a viral infection. If you’re wondering how viruses are spread, and what you can do to stay healthy, check out this new Let’s Talk Science backgrounder on Germs and How They Spread.


Thank you to the Let's Talk Science Challenge volunteer writers who provided content in this backgrounder.

Viruses (Updated) (2018)

This animated video (6:48 min.) from the Amoeba Sisters shows the structures of viruses and their life cycle.

Intro to Viruses

A basic introduction to viruses from Khan Academy, explaining their structures and their life cycle. 

Viruses: Molecular Hijackers (2017)

This video (10:01 min.) fromProfessor Dave Explains that  explains the parts of virus, how it infects cells, and how it replicates.


Freudenrich, C., & Kiger P. J. (2020). How viruses work. HowStuffWorks.

Morgridge Institute for Research. (2020). Virus structure.

Open Text BC. (n.d.). The viral life cycle.

Wagner, R. & Krug, R. (n.d.). Virus. Encyclopaedia Britannica.

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