Didier Raoult, a microbiologist at Aix-Marseille University in France, must be some sort of virus whisperer. He'd made various discoveries about the world of viruses over the years. In the latest study that he co-led with fellow microbiologist Bernard La Scola, for instance, they discovered that a type of giant virus called mimivirus has an immune system and can protect itself against other viruses. Raoult believes the study's findings are proof that mimiviruses, which typically grow inside amoeba, constitute a separate and fourth domain in the Woese system of classification. Under the Woese system, the three recognized domains of cellular life are Bacteria, Archaea (single-celled microorganisms) and Eukarya (cells with membranes and organelles).