What is a virus?
A virus is a micro-organism that can not grow or reproduce outside of a living cell. It typically consists of a nucleic acid molecule in a protein coat. Some viruses also have a membrane, like cells, with more proteins attached to it.
It may help to think of a virus as a robot. Robots only do what they are programmed to do. Virus-robots are programmed to multiply - they want to make more robots like themselves. But they need materials to make copies of themselves. So they find a suitable factory, invade it, stop it making whatever it made before, and use it to make robots. The robots coming out of this factory go to find more factories to change into robot-factories.
Genes in a box
Viruses come in many different shapes and sizes, but there are some things they all have in common:
• A genome*
• A container - capsid and maybe envelope
The virus is too small to carry all the molecular machinery required to replicate itself, which is why it needs a host cell to multiply. When it infects the host cell, the cell will be hijacked by the virus and turned into a factory for new virus particles. These particles will be released and go on to infect other cells.
The most basic viruses only have enough genome to encode their capsid and a genome multiplication protein (called polymerase). Those viruses are about 30 nm (millionth of a millimetre) in size. There are even smaller virus particles, called satellite viruses, that just have a coat protein - these need to use the polymerase of another virus. Examples of very small viruses are Tobacco Mosaic Virus (helical) and Tomato Bushy Stunt Virus (icosahedral) in plants, poliovirus in humans and bacteriophage MS2.The capsid is important to a virus because genomes are very fragile if they are loose in the environment. In addition, the capsid and envelope help the virus find and enter new host cells.
How do viruses spread?
Viruses do not move independently: they are dependent on currents or brownian motion to move to a new host cell. Many bacterial viruses lyse (break up) the host cell when enough new virus particles have been produced. The virus particles are released into the surroundings to find new host cells to infect.
In plants, the virus is often spread by a vector. The vector causes mechanical damage to the plant cells by biting. If insects like aphids drink sap from an infected plant, and then go to a different plant, they can spread the virus.
For animals, including humans, vectors can also play a role. Yellow fever virus and dengue virus are spread by mosquitoes. Other viruses, such as human immunodeficiency virus (AIDS), rabies and hepatitis viruses are spread by contact with blood or other bodily fluids. And of course colds and flu are spread by coughs and sneezes and handshakes! Unless there is mechanical damage to the cell (which will kill many cells anyway) the next challenge for the virus is to enter the cell itself. Animal viruses will usually use proteins on their own surface to bind to proteins on the cell surface. The cell will then be tricked to take up the virus, like a tiny Trojan horse. Some bacterial viruses actually inject their genome into the bacterial cell.
Why do viruses make you ill?
The "aim" of the virus (and all life!) is just to multiply its genome. Making you ill either helps it spread or is a reaction from your immune system, or a combination of both. Many human viruses initially produce "flu-like symptoms", which are all reactions of your immune system. Coughing, sneezing and diarrhoea help the virus spread by releasing more virus particles into the environment, so good hygiene is important! There are many viruses that don't make you ill. In fact, the human genome contains many bits of virus genes!
How do we find viruses?
The first virus was discovered in 1892. Leaves from a tobacco plant affected by Tobacco Mosaic Disease, were mashed up and the sap was filtered. People knew that bacteria (the smallest form of life known then) would not pass through the filter, but the filtered sap could still infect other plants. They called the infectious agent Tobacco Mosaic Virus (TMV). Since then, many other viruses have been discovered, but they couldn't be see until electron microscopes were invented.
Viruses are too small to see with even the most powerful light microscopes. To see viruses, we need to use an electron microscope, which uses a beam of electrons to take images. To read more about electron microscopes, click here.