The strangest life forms on Earth just got a lot stranger.

In 2003, Didier Raoult of Aix-Marseille University in France and his colleagues discovered a new kind of virus lurking inside single-celled protozoans.

Like other viruses, it couldn’t grow on its own, lacking the biochemical machinery to build proteins and genes. Instead, it had to infect host cells and use their material to produce new viruses.