Virus particles (green) within an amoeba (Image by Science)

"Mimivirus" is the biggest virus found so far, and was discovered in a sample taken from a water cooling tower in Bradford, UK, in 1992.

It has at least 900 genes, an enormous number for a virus, and its size is more like that of a bacterium.

It can be spotted through a good optical microscope - most viruses can only be visualised by electron microscopes.

In terms of DNA, it is approximately a fifth bigger than the virus previously considered to be the largest in the world.

No SARS link

Although it has been linked to pneumonia in humans, it is in no way related to the SARS virus currently sweeping the Far East.

Mimivirus (top) is as big as some bacteria (Image by Science)

A report about the discovery was published on Thursday in the journal Science.

Amoebas, large single-celled organisms, are commonly found in air-conditioning systems in large buildings, and often harbour various bacteria and viruses inside them, which can go on to infect people working in those buildings.

The researchers who examined Mimivirus, from the National Centre for Scientific Research in Paris, France, said that blood samples from people with pneumonia had revealed antibodies for this virus, suggesting that their immune systems had come into contact with it at some point.

They believe it is a virus because it lacks certain genes which are universal to bacteria - but contains others which are known to have key functions in viruses.