A virus can be infected by a smaller virus

Sputnik replicates inside the factory that a giant virus builds in an amoeba. Where Sputnik is present, the giant virus produces fewer and deformed particles. The small one is a parasite of the large one.

Giant viruses build a viral factory, a compartment in the host amoeba where their particles are assembled. In 2008 a second, much smaller particle was found in those factories and named Sputnik. It cannot infect the amoeba alone. It needs the giant virus to have opened the shop. Once inside, it uses the shop. The giant virus's output drops, and a share of what it does produce is misshapen. La Scola and colleagues called Sputnik a virophage: a virus whose host is another virus's replication programme.

The word is the arguable part. The observation is not. A particle with its own genes rides the machinery a larger virus set up, and the larger virus is the one that suffers. Later virophages have been found with other giant viruses, in other water. Life's stack of parasites does not end at the thing that has no cell. It ends, if it ends, at a size we have not looked under yet.