LONDON (Reuters) - Even viruses can go down with a viral infection, French scientists reported on Wednesday, in a discovery that may help explain how they swap genes and evolve so rapidly.

A new strain of giant virus was isolated from a cooling tower in Paris and found to be infected by a smaller type of virus, named Sputnik, after the first man-made satellite.

Sputnik is the first example of a virus infecting another virus to make it sick.

Bernard La Scola and colleagues from the Universite de la Mediterranee in Marseille reported in the journal Nature that Sputnik was able to achieve a remarkable degree of gene mixing by "looting" genes from its host virus and other organisms.

Viruses are already known to infect and sicken bacteria but this is the first example of a virus infecting one of its own kind.

The finding may shed light on how viruses mutate so quickly -- a feature that can make them difficult to tackle with drugs and vaccines.

It also lends weight to the argument that viruses are true living organisms, despite not having cells.

"There's no doubt this is a living organism. The fact that it can get sick makes it more alive," said Jean-Michel Claverie, a virologist at the CNRS UPR laboratories in Marseilles.