Pandoravirus dulcis and Pandoravirus salinus, the two Pandoraviridae's representatives known for now, are the largest viruses known to date, by their genome: these viruses have a Dna chain comprising between 1.9 and 2.5 millions base pairs, two times more than other giant viruses such as Mimiviridae, Megaviridae, and even more than Pithovirus sibericum, which holds the biggest virus record (actual size), being 50% larger than the Pandoraviridae.
The Pandoraviridae are kind of lonesome representatives of the world of giant viruses, providing a distinct family that has very unfamiliar structure and genome, compared to other giant viruses, which are relatively similar to each other. However, Pandoravirus salinus and Pandoravirus dulcis yet have been found in very different locations : the first was picked up in the seawater of the coast of Chile, the second in a pool of fresh water in Melbourne, Australia, suggesting that this type of giant viruses may occupy a very extensive habitat. So why have they not been discovered before? Because it seems like, as well as other giant viruses, they preferentially infect amoeba, these tiny unicellular organisms, so small that no one really paid attention to, until recently.
Discovered in 2013 by the two french researchers Chantal Abergel and Jean-Michel Claverie, the pandoraviruses yet have an unusual and surprising characteristic : they have a really huge genome for a virus class, comprising 1.9 (for dulcis) and 2.5 (salinus) millions base pairs (Pandoravirus salinus so is the actual "biggest" virus know to date), coding numbers of genes which are very different from those of others giant viruses (approximately only 7% of their genes were previously known). Actually, they show a genomic complexity that makes them basically look like prokaryotic parasites. It has indeed been suggested that they might be a missing link between viruses and simple life forms, or an unknown branch in the tree of species.