What is a Virus?

Viruses are tiny organisms that may lead to mild to severe illnesses in humans, animals and plants. This may include flu or a cold to something more life threatening like HIV/AIDS.

How big are viruses?

The virus particles are 100 times smaller than a single bacteria cell. The bacterial cell alone is more than 10 times smaller than a human cell and a human cell is 10 times smaller than the diameter of a single human hair.

Are viruses alive?

Viruses by themselves are not alive. They cannot grow or multiply on their own and need to enter a human or animal cell and take over the cell to help them multiply. These viruses may also infect bacterial cells.

The virus particle or the virions attack the cell and take over its machinery to carry out their own life processes of multiplication and growth. An infected cell will produce viral particles instead of its usual products.

Structure of a virus

A virion (virus particle) has three main parts:

  • Nucleic acid – this is the core of the virus with the DNA or RNA (deoxyribonucleic acid and ribonucleic acid respectively). The DNA or RNA holds all of the information for the virus and that makes it unique and helps it multiply.
  • Protein Coat (capsid) – This is covering over the nucleic acid that protects it.

Lipid membrane (envelope) – this covers the capsid. Many viruses do not have this envelope and are called naked viruses.


Viruses are not simply taken into cells. They must first attach to a receptor on the cell surface. Each virus has its specific receptor, usually a vital component of the cell surface. It is the distribution of these receptor molecules on host cells that determines the cell-preference of viruses. For example, the cold and flu virus prefers the mucus lining cells of the lungs and the airways.

How do viruses infect?

Viruses do not have the chemical machinery needed to survive on their own. They, thus seek out host cells in which they can multiply. These viruses enter the body from the environment or other individuals from soil to water to air via nose, mouth, or any breaks in the skin and seek a cell to infect.

A cold or flu virus for example will target cells that line the respiratory (i.e. the lungs) or digestive (i.e. the stomach) tracts. The HIV (human immunodeficiency virus) that causes AIDS attacks the T-cells (a type of white blood cell that fights infection and disease) of the immune system.

Life cycle of a basic virus

There are a few basic steps that all infecting viruses follow and these are called the lytic cycle. These include:

  1. A virus particle attaches to a host cell. This is called the process of adsorption
  2. The particle injects its DNA or RNA into the host cell called entry.
  3. The invading DNA or RNA takes over the cell and recruits the host’s enzymes
  4. The cellular enzymes start making new virus particles called replication
  5. The particles of the virus created by the cell come together to form new viruses. This is called assembly
  6. The newly formed viruses kill the cell so that they may break free and search for a new host cell. This is called release.

Further Reading

Last Updated: Jun 5, 2019

Dr. Ananya Mandal

Written by

Dr. Ananya Mandal

Dr. Ananya Mandal is a doctor by profession, lecturer by vocation and a medical writer by passion. She specialized in Clinical Pharmacology after her bachelor's (MBBS). For her, health communication is not just writing complicated reviews for professionals but making medical knowledge understandable and available to the general public as well.


Please use one of the following formats to cite this article in your essay, paper or report:

  • APA

    Mandal, Ananya. (2019, June 05). What is a Virus?. News-Medical. Retrieved on August 18, 2019 from https://www.news-medical.net/health/What-is-a-Virus.aspx.

  • MLA

    Mandal, Ananya. "What is a Virus?". News-Medical. 18 August 2019. <https://www.news-medical.net/health/What-is-a-Virus.aspx>.

  • Chicago

    Mandal, Ananya. "What is a Virus?". News-Medical. https://www.news-medical.net/health/What-is-a-Virus.aspx. (accessed August 18, 2019).

  • Harvard

    Mandal, Ananya. 2019. What is a Virus?. News-Medical, viewed 18 August 2019, https://www.news-medical.net/health/What-is-a-Virus.aspx.


  1. Ethan Mellor Ethan Mellor Ukraine says:

    The article is good. I would like to know more about viruses.

  2. maseera mirza maseera mirza India says:

    best webside thank you

  3. Ralph R. Zerbonia Ralph R. Zerbonia United States says:

    In this article's 2nd paragraph, headlined "How big are viruses?" are the following words:
    "The virus particles are 100 times smaller than a single bacteria cell. The bacterial cell alone is more than 10 times smaller than a human cell and a human cell is 10 times smaller than the diameter of a single human hair."

    May I point out that you really can't get smaller than one time the size of something at which point you disappear. What I assume is meant by the above paragraph is the following re-written set of words:
    "The virus particles are 1/100 the size of a single bacteria cell. The bacterial cell alone is smaller than 1/10 the size of a human cell and a human cell is 1/10 the diameter of a single human hair."
    "The Virus particle's size is 1% of that of a single bacteria cell. That bacteria cell is less than 10% of the size of a human cell and that human cell's size is 10%  of the diameter of a human hair. So that makes the virus particle's size approximately 1/1000th the size of a human hair!" (Hope I got all the math right there! It's 1% of 10% of 10% of the diameter of a human hair)
    Remember if I subtract one whole you (1 time smaller) there is nothing left. You want to speak about fractions, not whole numbers when speaking about something smaller. Unless of course it somehow is possible for the item being described to actually go into negative numbers. Physical items don't. You can only have one less of a physical item.

    I know, I am feeling very much like a word Nazi, but I only do it for this particular peeve.....

  4. Stephen Spink Stephen Spink United Kingdom says:

    I wonder if a prolonged burst of low voltage electricity would affect the virus ability to function.

  5. Russell Wuertz Russell Wuertz United States says:

    Modern Latin is what the state Universities use.  Since the 16th century.   Yale would be different using Old Latin or Middle Latin, and is not what anyone has classes for.

    The Modern Latin meaning for "Virus" is a slimy liquid, or in my opinion, the reason for a pimple, chicken pox, small pox, cow pox.  Nitric acid, has been a problem ever since we did what was not allowed and pumped up crude oil.  The nitric acid from coal oil is the worst of all the acids we have, without the word cataylst being what we know, and how much it does not dilute, but comes back into its origional form and strength.  

    The experts eliminating this acid from effecting us today is the reason our teeth don't rot, and our children all don't have acne.

  6. Prettiecandie Lemo Prettiecandie Lemo Namibia says:

    virus .. I am looking forward to learn a lot on virus

  7. Bassem K Bassem K U.A.E. says:

    it's 1/10,000th not 1/1,000

  8. Snaz'n Gamer Snaz'n Gamer United Kingdom says:

    useful for school but the aids news letter  isn't aproporiot for young people

The opinions expressed here are the views of the writer and do not necessarily reflect the views and opinions of News-Medical.Net.
Post a new comment
You might also like... ×
Smartphone ‘virus scanner’ could help governments contain viral outbreaks