The debate about what counts as a living thing is fuelled today by the discovery of the first virus that is able to fall "ill" by being infected with another virus.
Viruses are glorified scraps of genetic code that are exquisitely designed to pirate a host to reproduce: the common cold virus needs cells in the nose and respiratory tract to reproduce, before being spread with a sneeze.
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But the discovery of a giant virus that itself falls ill through infection by another virus seems to suggest they too are alive, highlighting how there is no watertight definition of what exactly scientists mean when they refer to something as "living".
"There's no doubt, this is a living organism," the journal Nature is told by Prof Jean-Michel Claverie, director of the Mediterranean Institute of Microbiology in Marseilles, part of France's basic-research agency CNRS. "The fact that it can get sick makes it more alive."
When the giant virus infects a host cell, an amoeba, they create a huge structure within the host, like a transient cell, that makes more viruses.
Thus, he said, the virus has the same role in its life cycle as a sperm does in the human life cycle. And it is not a surprise that this transient parasitic cell is itself vulnerable to viruses.
The extraordinary discovery that the giant virus suffers infections of its own is reported in Nature by Prof Bernard La Scola at the Université de la Méditerranée, Marseille, and colleagues.
They used an electron microscope to look at cells infected with a new strain of mimivirus, the largest known virus, which was first recognised by Prof Claverie and colleagues five years ago.
Prof La Scola and his colleagues were surprised to spot a smaller type of virus attached to the virus-making factory inside infected cells. The new virus - Sputnik - was unable to infect cells by itself but seemed to hijack the larger to achieve its infectious aims.
The team suggests that Sputnik (after "travelling companion" in Russian) is a 'virophage', much like the bacteriophage viruses that infect and sicken bacteria.
The giant virus was first isolated in amoebae from a cooling tower in Bradford. Since then, genetic studies of ocean waters have indicated that these giant viruses are very important, and may play a crucial role in regulating the population of plankton as well as influence the climate.
A genetic study of ocean water has revealed an abundance of genetic sequences closely related to giant viruses, leading to a suspicion that they are a common parasite of plankton.
By regulating the growth and death of plankton, giant viruses - and satellite viruses such as Sputnik - could be a major influence on ocean nutrient cycles and climate.
"These viruses could be major players in global systems," Nature is told by Prof Curtis Suttle, an expert in marine viruses at the University of British Columbia in Vancouver.
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