The married virologists at Aix-Marseille University had made a career of it. But pithovirus, which they discovered in 2013 in a sample of Siberian dirt that had been frozen for more than 30,000 years, was more bizarre than the pair had ever imagined a virus could be.

In the world of microbes, viruses are small—notoriously small. Pithovirus is not. The largest virus ever discovered, pithovirus is more massive than even some bacteria. Most viruses copy themselves by hijacking their host's molecular machinery. But pithovirus is much more independent, possessing some replication machinery of its own.

Pithovirus's relatively large number of genes also differentiated it from other viruses, which are often genetically simple—the smallest have a mere four genes. Pithovirus has