As a general rule-of-thumb, eukaryotic cells (e.g., amoebae or human body cells) are about 1,000 times bigger than bacteria, and bacteria are about 10-100 times bigger than viruses. But in biology, there is rarely such a thing as an inviolable rule. For example, two types of giant amoeba-infecting viruses, known as Megaviridae and Pandoravirus, are so large that they are comparable to bacteria in terms of physical size and genome length.

Now, after poking around in the Siberian permafrost, a team of French and Russian scientists have reported the discovery of a third. The virus, which they named Pithovirus, resembles a mishmash of the two previously known giant viruses. And remarkably, it is 30,000 years old!

Panel A depicts a close-up of the virus's envelope and inner membrane. The very top of the virus contains what looks like a "cork." Panel B depicts a cross-section and a longitudinal-section, respectively. The arrow points to what appears to be a long, tubular membrane connected to the cork.

Panel C depicts the hexagonal cork, as seen from the top of the virus. (If the cork's tiny holes bother you, perhaps you have trypophobia.)

The most interesting part of the research, though, isn't the discovery of a new virus. Instead, it is that the virus was buried 30,000 years ago and revived when it was cultured in the presence of amoebae, which it promptly infected. The authors ominously warn that previously undisturbed environments -- even ancient ones -- could be sources of emerging infectious diseases for humans and animals.

It should be noted, however, that it's not necessary to dig around in the dirt for scary new viruses. The MERS virus, which has already killed several dozen people, made its global debut in 2012 and is linked to contact with camels. And influenza evolves so rapidly that pandemics are a constant threat.

Still, the authors rightly conclude that their "results thus further substantiate the possibility that infectious viral pathogens might be released from ancient permafrost layers exposed by thawing, mining, or drilling." Indeed, that is a tad unsettling.

Source: Matthieu Legendre et al. "Thirty-thousand-year-old distant relative of giant icosahedral DNA viruses with a pandoravirus morphology." PNAS Early Edition. Published online before print: 3-Mar-2014. doi: 10.1073/pnas.1320670111