Following this definition of life, viruses typically met the latter two tests, but failed the first as they only produced a few structural or protective proteins to encase themselves.  This was a primary justification in classing them as nonliving.  However, recent discoveries have troubled this comfortable notion of the solid boundary between life and nonlife.  First, a variety of parasitic bacteria have been discovered, many of which lack the key protein enzymes needed to survive outside their host -- another bacteria.

Also, giant viruses have been discovered, which in addition to infection related proteins, make a variety of other proteins which help it carry out other processes.  On a level of complexity they can surpass the microbacteria, but they're clearly related to other much smaller viruses.

Now a new piece of evidence supporting that viruses may be somewhat "alive" has been added -- viruses can catch a virus. 

The discovery began more than a decade prior when researchers found a massive parasite in an amoeba from a cooling tower in Bradford, UK.  The little creature was frozen, as it was thought to be run-of-the-mill parasitic bacteria.  However, upon closer inspection, years later the scientists recognized it as a virus, with a gigantic genome, capable of encoding over 900 proteins.  The virus was named Acanthamoeba polyphaga mimivirus (for mimicking microbe).  It was over three times bigger than any previous virus. 

The discovery brought great excitement to some nontraditional biologists who had long believed viruses to be living.  Says Eugene Koonin of the National Center for Biotechnology Information in Bethesda, Maryland, "It was the cause of great excitement in virology.  It crossed the imaginary boundary between viruses and cellular organisms."

Now Professor Koonin, Jean-Michel Claverie, a virologist at the CNRS UPR laboratories and Didier Raoult at CNRS UMR, made an even more shocking discovery.  The team in 2003 discovered an even bigger virus.  They named this one mamavirus.

The shocker came when they found that a smaller virus with just 21 genes was associated with the new mega-virus and was infecting it.  While the main virus infected the amoeba, hijacking the amoeba's enzymes and structure to make a protein factory, the tiny virus, which researchers named "Sputnik" set to work hijacking this factory and making copies of itself.

The result was the mamavirus got more than just a bad cold -- it produced fewer and deformed mamaviruses, effectively making it less infective.  This relationship of a viral parasite sickening a host is one only expected by something living, further evidence that the big viruses might be "alive".

Says Jean-Michel Claverie on the mamavirus, "There’s no doubt this is a living organism.  The fact that it can get sick makes it more alive."

Mr. Koonin adds, "It infects this factory like a phage infects a bacterium.  It’s doing what every parasite can — exploiting its host for its own replication."

Intriguingly the little virus has genes similar to those used by the mamavirus and mimiviruses for reproduction.  This leads some researchers to speculate the virus could have been created by a failed reproduction by the big viruses.  This also supports the idea of the big viruses as being alive, as a prevalent theory for the origin of viruses was that they came from misreplicated bacterial DNA.  Further the little virus can transform genes between big viruses, similar to horizontal-gene-transfer in bacteria.

The new finding may have a big impact on global biology.  In plankton blooms genetic sequences have been found similar to those in the big viruses.  These blooms may be teaming with big viruses, which would likely have been destroyed by sample collectors' bacterial filters.  By impacting the life and death of plankton the big viruses could impact not only ocean nutrient cycles, but the global climate itself. 

Curtis Suttle, an expert in marine viruses at the University of British Columbia in Vancouver states, "These viruses could be major players in global systems.  I think ultimately we will find a huge number of novel viruses in the ocean and other places.  It emphasizes how little is known about these organisms — and I use that term deliberately."

The full study on the topic of large plankton viruses can be viewed here, while the study on the mamavirus can be viewed here.