Among pathogens, viruses are unique in their collective ability to infect all types of organisms. There are plant viruses, insect viruses, fungal viruses, and even viruses that infect only amoeba and bacteria. Now a group of researchers at the Centre National de la Recherche Scientifique in France has made the startling discovery that even some viruses can have viruses.

In a paper in Nature last month, the group described how they identified a giant mimivirus in a cooling tower in France. Mimiviruses are the largest viruses known to exist — so big they are visible under a normal optical microscope (usually much higher resolution electron microscopes are needed to view them). The new virus, large even by mimivirus standards, was appropriately named "mamavirus."

In the same cooling tower, the French group also discovered a second, tiny virus that infects the giant mamavirus. This they named "Sputnik."

Sputnik is unusual because it is the first virus ever discovered that is a parasite of another virus. When it reproduces in a cell infected by the larger virus, its action impairs the reproduction of mamavirus particles. The group sequenced Sputnik's genetic code and discovered that a number of its gene sequences are similar to those found in a massive survey of genetic material taken from oceans all over the globe. This suggests that a whole class of viruses might exist that infect other viruses.

Moreover, the discovery rekindles the debate over whether viruses are alive. Viruses are often not considered living organisms because they lack their own cellular structure and the ability to metabolize food — traits common to all known forms of life. The ability of one virus to infect another calls into question the possibility that viruses should be considered forms of life — though this is no great revelation for some.

"For me it is does not change anything," says Bernard Lascola, who was the first author on the paper. "I always considered that viruses are alive."

Inside Science News Service is supported by the American Institute of Physics.