The Mimivirus is the largest virus known, so large that it was initially mistaken for a bacterium (okay, still really small by our standards!) living inside of an amoeba. Since 2003, we've known it is actually a giant virus infecting an amoeba. Now we know that Mimi can get sick too. A different and remarkably small virus, called Sputnik, makes its living by infiltrating the Mimivirus factory and reproducing faster, a process that also leads to the formation of fewer, and "sicker" (that is to say, deformed) Mimiviruses. Since Sputnik hurts the Mimivirus, and is incapable of infecting an amoeba without the Mimivirus, it could be considered a viral parasite of another virus.
This ecological relationship is intriguing because viruses fall into a tricky category between the living and the nonliving. We think of single cells as the simplest form of life, but viruses don't even have a cell. Other hallmarks of the living, like eating and being able to reproduce with others of your species or alone, are conspicuously absent. Reproduction depends on hijacking a cell, and using its machinery and energy to set up a virus factory that pumps out new viral particles. But just like living organisms, viruses do have genes and are evolving through natural selection.
The Mimivirus has a lot of genes for a virus (911) and some of them seem to come from shuffling genes with amoeba hosts and other microbes. Sputnik only encodes 21 genes, which is why it requires a helper like Mimi to make multiplication possible. The cool thing about those 21 genes is that they are closely linked to a variety of other viruses that infect really diverse organisms - bacteria, eukaryotes, and archaea. Where do viruses like Sputnik come from? And what is the ecological relationship between small and large viruses, if any?
Evidence might be in the ocean! Some of the genes identified as being most similar to Sputnik's came from Craig Venter's Global Ocean Survey. The authors of the study (La Scola et al. 2008, in Nature) point out that large viruses, similar to the Mimivirus, are abundant in the ocean. The small viruses in the sea are able to quickly multiply like Sputnik. Sometimes both small and large viruses can be found in marine amoebae. Large and small viruses could evolve to trade genes and depend on each other in different ways. And small viruses could shuttle genes between large viruses.
Learning more about the ecology of viruses may or may not get us closer to understanding whether they're alive, but could help us understand some big picture processes. Viral infection is an important regulator of marine microbes... and marine microbes are responsible for some major chemical fluxes that regulate the climate on our planet. But more on that another time!