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Researchers have discovered the first virus to infect another virus, according to a study appearing tomorrow in Nature. The new virus was found living inside a new strain of the viral giant, mimivirus.

"This is one parasite living on another parasite, which is really fascinating," Michael Rossman, microbiologist at Purdue University, who was not involved in the study, told The Scientist.

Didier Raoult and colleagues at the Universitee de la Mediterranee in Marseilles, France, discovered Mimivirus in 2003 from a water cooling tower in the UK. It primarily infects amoeba, although antibodies have been found to the virus in some human pneumonia cases. It measures in diameter about 400 nanometers (nm), while medium-sized viruses such as adenovirus and HIV measure closer to 100-200 nm.

In this study, Raoult's team found a new strain of mimivirus in water from a cooling tower in Paris. This new strain was even larger than mimivirus, so the researchers named it mamavirus. To their surprise, while examining the new strain with electron microscopy they saw a smaller virus attached to mamavirus. This small virus comprises only 20 genes (mimivirus has more than 900 protein-coding genes) and the researchers named it Sputnik.

The team quickly set to work to see what effect Sputnik was having on mamavirus. They found that Sputnik infects the replication machinery in mamavirus and causes it to produce deformed viral structures and abnormal capsids, where viral genetic information is stored. It had a similar effect on mimivirus. Because Sputnik's behavior so closely resembles what bacteriophage do to bacteria, the researchers called the new type of virus a virophage, and suspect it may represent a new virus family.

The researchers found that Sputnik's genes shared homology with genes from all three domains of life: Archea, bacteria, and eukaria. Some of the genes were homologous to novel sequences that scientists previously detected in a metagenomic study of ocean water. This supports the idea that Sputnik is part of a larger family of viruses, Bernard La Scolla, researcher at the Universitee de la Mediterranee and first author on the paper, told The Scientist.

The size of a virus may dictate whether it can be infected by smaller viruses such as Sputnik, he added. For this reason, viruses that affect humans -- like HIV and influenza -- are likely too small to be infected by Sputnik-like viruses, said Rossman.

La Scolla added he is sure that there are other giant viruses yet to be identified in the world, but they won't necessarily be infected by smaller viruses. "We need to be lucky to find another Sputnik."