A virus that has lain dormant while frozen in the arctic ice for tens of thousands of years has become infectious again, scientists have found.
The new virus was found in Siberian permafrost and belongs to a class of giant viruses that were discovered ten years ago.
Measuring 1.5 micrometers in length, the Pithovirus is the biggest that has ever been found. A micrometre is one millionth of a metre.
So-called "giant viruses" contain a large number of genes compared to common viruses such as influenza or Aids, which contain approximately ten genes.
In contrast to "regular" viruses, they can easily be easily seen under a microscope.
Researchers from the National Centre of Scientific Research (CNRS) at the University of Aix-Marseille in France, say that the Pithovirus has almost nothing in common with the giant viruses that have previously been observed.
That makes it the first member of a new virus family. They also found it infects amoebas, a single celled organism, but is not a threat to humans.
However, according to the researchers, the revival of the virus could mean there may be other threats to human or animal life hidden in the permafrost.
They added that new viruses could be exposed as global warming or industrial exploitation of the polar regions causes permafrost to thaw.
One of the authors of the study Professor Jean-Michel Claverie told the BBC: "It is a recipe for disaster. If you start having industrial explorations, people will start to move around the deep permafrost layers. Through mining and drilling, those old layers will be penetrated and this is where the danger is coming from."
Professor Claverie said that ancient strains of the smallpox virus could pose a risk in this way as its replication process is similar to thePithovirus.
The virus was found buried 100 feet underground in the Chukotka region of north-eastern Siberia
Scientists added some of the permafrost to amoeba colonies to see if it contained infectious material.
The amoebas began to die and scientists discovered that the virus was multiplying inside the amoeba cells before bursting them.
Professor Chantal Abergal, another of the study's authors said that the discovery could prove to be very important in the effort to eradicate diseases.
She said: The fact that we might catch a viral infection from a long-extinct Neanderthal individual is a good demonstration that the notion that a virus could be "eradicated" from the planet is plain wrong, and give us a false sense of security.
She added that the new virus could also help answer key questions about our planet:
"The discovery of yet another new family of giant viruses is a clear indication that we still know very little about the biodiversity on our planet, at least in unexplored environment."
"These giant viruses exhibit a very large proportion of genes without any resemblance with the millions that have been characterised in known organisms. Part of what we don't know might turn out to contain the explanation to fundamental questions such as the origin of life on our planet."