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1.13: Viruses

Difficulty Level: At Grade Created by: CK-12
Practice Viruses
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What causes the common cold?

That miserable cough and runny nose is caused by one villain: a virus. Viruses come in many different shapes, including the prickly balls you see here. They are so tiny that they can only be seen with a very powerful microscope.

What is a Virus?

We have all heard of viruses. The flu, the common cold, and many other diseases are caused by viruses. But what is a virus? Do you think viruses are living? Which domain do they belong to? Bacteria? Archaea? Eukarya?

Are Viruses Alive?

The answer is actually “no.” A virus is essentially DNA or RNA surrounded by a coat of protein ( Figure below ). It is not made of a cell , and cannot maintain a stable internal environment ( homeostasis ). Recall that a cell is the basic unit of living organisms. So if a virus is not made of at least one cell, can it be living? Viruses also cannot reproduce on their own—they need to infect a host cell to reproduce. So a virus is very different from any of the organisms that fall into the three domains of life.

Though viruses are not considered living, they share two important traits with living organisms. They have genetic material like all cells do, and they can evolve. As the process of evolution has resulted in all life on the planet today, the classification of viruses has been controversial. It calls into question the very definition of life.

Drawing of viruses attacking a cell

These little "alien" looking creatures are viruses, and these specific viruses infect Escherichia coli bacteria. Shown is a representation of viruses infecting a cell. The virus lands on the outside of the cell and injects its genetic material into the cell.


Viruses infect a variety of organisms, including plants, animals, and bacteria. Once inside the host cell, they use the cell’s own ATP (energy), ribosomes , enzymes , and other cellular parts to make copies of themselves. The host cell makes a copy of the viral DNA and produces viral proteins. These are then packaged into new viruses. So viruses cannot replicate or reproduce on their own; they rely on a host cell to make additional viruses.

Viruses and Human Disease

Viruses cause many human diseases. In addition to the flu and the common cold, viruses cause rabies, diarrheal diseases, AIDS , cold sores, and many other diseases ( Figure below ). Viral diseases range from mild to fatal.

Cold sores are caused by a herpes virus

Cold sores are caused by a herpes virus.


  • A virus is composed of DNA or RNA surrounded by a coat of protein.
  • Viruses are not considered living things because they cannot reproduce on their own, and they are not comprised of cells.

Explore More

Use the resources below to answer the questions that follow.

Explore More I

  1. How do viruses reproduce? How does this differ from other organisms?
  2. What kinds of nucleic acids can viruses have?
  3. Explain one of the theories as to how viruses came to be.
  4. What is the importance of the "envelope" to a virus? What is the envelope made of?
  5. What is a difference between the lytic cycle of a virus and the lysogenic cycle?

Explore More II

  1. What is one way a flu virus can kill a human?
  2. Do mutations make viruses more deadly? Why or why not?


  1. Is a virus a living thing? Why or why not?
  2. Name four examples of human diseases caused by a virus.




Acquired immune deficiency syndrome, which is a fatal condition, unless treated with proper medications, caused by the human immunodeficiency virus (HIV).


Adenosine triphosphate; a usable form of energy inside the cell.


Basic unit of structure and function of a living organism; the basic unit of life.


Protein that speeds up chemical reactions.


Process in which something passes to a different stage, such as a living organism turning into a more advanced or mature organism; the change of the inherited traits of a group of organisms over many generations.
genetic material

genetic material



Maintaining a stable internal environment.
host cell

host cell

Cell infected by a virus.


To make a copy of, as in viral replication or DNA replication.


Cell structure on which proteins are made; not surrounded by a membrane; found in both prokaryotic and eukaryotic cells.


Non-living particle consisting of RNA or DNA surrounded by a protein coat.

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Difficulty Level:

At Grade

Concept Nodes:


7 , 8

Date Created:

Nov 29, 2012

Last Modified:

Feb 18, 2015
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