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  • It doesn’t breathe, it doesn’t eat, it doesn’t excrete, and it doesn’t grow – so it can’t be alive, can it? It hijacks a living cell and uses it to produce so many copies of itself that it bursts the cell – so it can’t be dead, can it?

    What is it? A virus.

    Viruses come in many shapes and sizes and infect every living thing.

    Viruses are very, very small – about one 500th the size of a single skin cell. Comparing a virus to the size of a flea is like comparing a person to the size of Mt Everest. If a skin cell was the size of an A4 page, a virus would be the size of a 10 cent piece.

    Viruses are very simple, with a relatively short piece of genetic code inside a packaging of protein called a capsid. When the virus infects a cell, it inserts its genetic code into the cell and takes over the machinery of the host cell to make lots of copies of its genetic code and proteins for its capsid. The new viruses are then assembled inside the host cell and eventually burst the cell and kill it. Viral destruction of cells causes disease.

    Different viruses infect different types of cells, which is why each causes its own unique disease. For example, HIV (the AIDS virus) infects immune cells, which is why patients with the HIV virus are unable to resist other infections and cancers. The cold virus infects the cells in the back of your nose and throat, which is why you get a sore throat, sneeze a lot and get a runny nose.

    There are even a whole group of viruses that infect bacteria called bacteriophage. The term virus comes from the Latin for poison.

    Grouping viruses

    Viruses are grouped according to the type of genetic code they carry – either DNA or RNA – and then by the shape of their capsid. The diagrams above shows the relative size and appearance of some virus families. The family name has been given as well as one of the human diseases caused by a virus from that family.

    A virus is a simple structure yet it can be deadly. Viruses replicate quickly and can go undetected by our immune systems due to mutation or reassortment. This leads to new virus strains. Explore this further in the article Virus strains.

    Nature of Science

    Scientists are discovering how viruses affect cells, but this changes with time as the viruses mutate. Often, scientific knowledge is tentative or based on our knowledge at the time the observations were made.

    Activity ideas

    Exploring microorganisms asks students to research and summarise the key features of bacteria, viruses and fungi.

    Make an adenovirus uses a template to make a model of an adenovirus and asks students to investigate the different shapes of different viral diseases and the similarities and differences between viral families.

    Spreading diseases demonstrates how a viral disease can spread through a population, even if an individual only comes in contact with three other people.

    This article is about coronaviruses, a group of viruses that cause upper respiratory illnesses in humans. In 2020, the COVID-19 virus caused a global pandemic.


    We have curated content in this Viruses and the immune system collection – it explores viruses and how our immune system fights them. Log in to make this collections part of your private collection. Click on the copy icon, and then you can add additional content and notes and make other changes.

    Useful links

    In this Science on a Napkin video Hannah Brightley interviews Dr Heather Hendrickson (Massey University) to learn more about bacteriaphages and how they could be used to help fight honeybee disease American FoulBrood (AFB).

    In this Radio NZ Our Changing World programme Investigating the virosphere discover the work being undertaken on a range of native New Zealand species to find out what viruses are associated with them. The goal is to build on our knowledge of virus diversity and to help us figure out how viruses jump to new hosts.

      Published 9 November 2008, Updated 4 February 2020 Referencing Hub articles
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