Viruses come in all sizes and shapes, but a new organism found in Siberia is the largest one ever discovered. 

Pithovirus sibericum measures 0.00002 inches wide by 0.00006 inches long. That may not sound like much, but in the world of viruses, this variety is the largest ever seen by researchers. The second-largest virus known pandoraviruses, which have a length just two-thirds that of the Pithovirus.

Pandoraviruses were discovered earlier by the same team, and are considered a second distinct form of virus from all other varieties. 

"While initiating a survey of the Siberian permafrost, we isolated a third type of giant virus combining the Pandoravirus morphology with a gene content more similar to that of icosahedral DNA viruses," wrote the researchers in the article announcing the results. 

While pandoraviruses have up to 2,500 genes, pithoviruses are much simpler, containing just 500 genes. This is still not the simplest virus in the world today. The organism that causes HIV has just 12 genes. Humans carry 20,000 to 25,000 genes capable of coding proteins. 

This new discovery is thought to be a leftover from 30,000 years ago, and has not encountered humans - or animals - since that time. Since the virus has been released from the permafrost, there is a chance it could get out into the world once more. Fortunately, the virus only appears to attack amoebas. Human and rat cells were found to be immune from infection by the giant virus. 

Viruses are the simplest organism which could constitute life. Genetic code is carried on DNA, and those genes are wrapped in protein casing. Because their structure is so simple, and they carry out so few functions, biologists argue over whether viruses can be considered to be living organisms. 

Discovery of the virus was headed by Jean-Michel Claverie and Chantal Abergel of Aix-Marseille University, located in Marseille, France.

There is little chance the virus comes from outer space, but it could represent a whole new domain of lifeform, previously unseen. Pithovirus may serve as a warning for the future. Although this virus can not infect humans, there may be other, more dangerous, viruses hidden in ices of Siberia and elsewhere. As global temperatures rise, researchers are concerned new diseases, long hidden, may come back to life. 

"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 gives us a false sense of security," researchers wrote in a press release. 

Details of the study were recording in the Proceedings of the National Academy of Sciences

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