Rights: The University of Waikato Published 18 June 2008 Download

Dr Wendy Williamson of ESR’s water management team explains that viruses are extremely small infectious particles that need to infect cells in order to replicate themselves. Viruses infect every form of cellular life, from plants and animals to bacteria. Some viruses are capable of causing disease in the organisms they infect.


A virus is a submicroscopic infection agent, so that means it’s really, really small, so it’s submicroscopic. We can't see them with a normal light microscope – you have to use very high resolution microscopy, which is electron microscopy, to see them. They are infectious, they will make people sick. Their entire life-cycle is based on infecting a cell, so they infect all cellular life. They cannot exist on their own, they cannot replicate – that is make more of themselves – unless they are within another cell, that is, they are parasitic. They will infect all cellular life, that is, animals, plants and bacteria. And so within the viruses, every one of those groups of animals will have viruses that will be specific to them, and are very unlikely to infect others. Viruses that infect bacteria are unlikely to infect plants, or infect animals and all the other combinations. They don't feed – that is one of the things that distinguishes viruses from other organisms is that they are almost, like, on the edge of life, they are almost not alive and not dead. So they don't take in nutrients and they do not grow and increase in biomass in the normal way that we would think of a plant, a bacterium, or an animal increasing in size by uptake of nutrients. They simply replicate by hijacking all the machinery within another cell. It has a very small amount of its own genetic material that takes over the cell, and it stops that cell doing what it was doing before and makes that cell begin to make more of the virus, and only the virus, until the cell ruptures and releases millions of copies, and then those copies of the virus will then go and infect all the cells just adjacent to itself, and so the infection continues. And that is why you can get very rapid and very serious disease because you get usually a lot of tissue damage, or the cells actually try and make different compounds to protect themselves, and you can get a lot of immune response reactions, which are part of what also make us sick from viruses. Antibiotics do not work against viruses, and that's because viruses do not grow. There are some anti-viral therapies available, and many more being developed, especially for difficult diseases like HIV and for bird flu – things like that. Any therapy that is going to interfere with a virus is, by its very nature, also going to interfere with normal cellular activity, so therefore there is always a downstream risk of something going wrong.

Susan Lim, ESR
Dr Richard Tilley
Influenza, Eastern equine encephalitis, Ebola virions, Smallpox and Lassa virus micrographs from F.A.Murphy, School of Veterinary Medicine, University of California, Davis.