A massive virus has been found lurking in a British water cooling tower. The virus lives in single-celled organisms called amoebae and may be able to infect humans.

The virus is so large that at first researchers mistook it for bacteria. Most viruses can only be seen with electron microscopes but this one was spotted through a high quality optical microscope.

The virus was dubbed Mimivirus, short for "mimicking microbe." Didier Raoult at the Mediterranean University in Marseille and his team weren't able to detect bacterial-specific genes in the virus.

Mimivirus has at least 900 genes, an enormous figure for a virus. The team compared its genes to other viruses and found it is related to other large viruses, such as smallpox.

The virus has been classified as the first of a completely new virus family, the Mimiviridae. Its identity needs to confirmed.

British microbiologist Tom Rowbotham took the initial sample of Mimivirus from the air conditioning unit of a water cooling tower in Bradford, northern England, in 1992.

Amoebae, large single-celled organisms, are commonly found in air conditioning systems and temperate waters. Amoebae feed on germs and can harbour bacteria and viruses such as Legionella and Chlamydia pneumoniae, which can cause respiratory disease.

Raoult said blood samples from people with pneumonia showed antibodies for the virus. It has no connection to SARS virus currently spreading in the Far East, he said.

The study appears in Friday's issue of the journal Science.