What Is A Virus?

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   Imagine something that cannot grow or respond,

but can reproduce itself. Viruses show almost

none of the characteristics of living things and

are not classified as living. Viruses can only

reproduce if they are inside a living cell.

 

Image: Electron microscope image of norovirus particles.Noroviruses cause the “stomach flu”.  CDC/Chales D. Humphrey http://phil.cdc.gov/phil/details.asp

 

   Viruses are made up of DNA or RNA (its genetic code) and a protein coat. Sounds simple, right?  But, the few structures viruses have are actually complex chemical compounds which allow viruses to infect many different kinds of organisms and to cause disease.

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Learning Objectives  

By the end of this lesson students should be able to:

• Describe the structure of viruses.

• Name four diseases caused by different types of viruses.

• Explain how viruses reproduce and cause disease.

• Explain why viruses are not classified as living even though they can reproduce.

 

Vocabulary  

capsid—the protein shell of a virus which encloses the genetic material.

 

coronavirus—group of viruses named for the crown-like spikes on the surface of the enveloping protein coat; cause upper respiratory tract illnesses.

DNA—the genetic code of some viruses, such as the hepatitis virus and HPV.

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host—an organism on which another organism lives.

 

RNA—the genetic code of some viruses, such as influenza, mumps, and the common cold.

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virology—the study of viruses.

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virus—an infectious agent consisting of genetic material (DNA or RNA) and a protein coat.

 

Virus Structure   

                                                  Viruses are made up of a protein coat which                                                        encloses either DNA or RNA, the genetic code                                                      for the virus. Some viruses also have a                                                                      membrane that covers the protein coat, but not                                                    all viruses have this membrane.

Image: Model of an influenza virus. Credit: NIH

 

  Other features of some viruses make them well adapted for causing disease. For example, the protein coat is made up of different types of proteins arranged in different shapes on the surface. For example, some protein coats have spikes and tails. Spikes and tails help the virus attach to host body cells.

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   Think about how you get infected with a virus. You come into contact with a virus when someone coughs or sneezes or when you touch something that has the virus on it. The virus enters your body and goes into your cells. Once inside the cell, the viral DNA or RNA can be replicated (duplicated) and thousands of new viral particles are produced. The cell dies and the new viral particles are released to infect even more of your cells. You get sick.

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How Do Viruses Multiply?

   Once inside the body, viruses are able to infect cells and cause disease. Viruses multiply slightly differently depending on the type of virus, but almost all go through the same five steps:

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1) Attach:  The virus attaches to a host cell. Any cell will do as long as the virus can attach to specific chemical groups on the cell surface.

 

2) Invade: The entire virus or just its genetic material penetrates the cell and enters the cell’s cytoplasm (gel-like substance inside the cell membrane).

 

3) Copy: The viral DNA or RNA uses the host cell to produce thousands of copies of viral genetic code.

 

4) Assemble: The new viral genetic code and proteins form new viral particles.

 

5) Release: New viral particles are released from the cell, the cell dies, and the new particles go on to infect other healthy host cells.

 

Living or Non-Living?

   Scientists have debated over whether viruses are living or non-living for decades. Why?  It is pretty obvious if something is living, right? Not for viruses.

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   While viruses can reproduce, a characteristic of living things, they can only reproduce inside a living cell. Viruses do not have most of the characteristics of living things. They do not grow, they do not take in food, and they do not respond to their environments. While reproduction is only a characteristic of living things, scientists do not classify them as living because they do not have any other living characteristics.

  

   Viruses are often classified by their shapes. For

example, the coronaviruses appear to have a halo

surrounding the protein coat. The corona-like

appearance results from protein spikes on the surface

of the virus.

Image: Electron microscope image of the coronavirus that causes MERS (Middle

East Respiratory Syndrome). Content Provider: NIAID http://phil.cdc.gov/Phil/details.asp

 

Viruses as Pathogens

 

   Viruses are efficient pathogens (disease causing structures).  You are probably already familiar with many of the diseases viruses cause and have probably been immunized against quite a few. Measles, mumps, chickenpox, rabies, influenza and the common cold are all viral diseases.

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   Each viral disease is spread to animals or people in a way specific to that virus. Knowing how the virus is spread (transmitted) is an important way to avoid getting the virus and disease. Many viruses are dangerous and threaten lives.

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Read below for examples of viruses that can have deadly consequences.

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Influenza

   Millions of people get the flu every year.  While

some people have mild illness, others have severe

complications that can result in hospitalization or

death.

Image: An influenza virus. Credit: Frederick Murphy http://phil.cdc.gov/phil/

details.asp

 

   There are three types of influenza viruses: A, B, and C. Influenza types A and B cause human seasonal flu outbreaks almost every year. Influenza type C viruses cause only mild respiratory illness and do not cause widespread outbreaks or epidemics.

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   Each year, a vaccine is produced after scientists predict which flu types may infect the population that year. The best way to prevent getting influenza is to get a flu vaccine.

 

Read more about vaccines in "How Vaccines Prevent Disease" in this module.

 

Covid-19                        

   COVID-19 is a new type of coronavirus that appeared in late in 2019 in Wuhan, China. It quickly spread, not only in China, but across the world.

  

   COVID-19 is spread (transmitted) when an infected people sneezes or coughs. Infected droplets in the air enter another person’s nose, mouth or eyes. People can also get infected if they touch an infected droplet that is on a surface and then touch their nose, mouth or eyes.

  

   As scientists study the virus and learn about how it is transmitted and how long the virus can live on surfaces and in the air, we will be better able to avoid becoming infected.

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