Carl Zimmer in his excellent blog, The Loom:
For years, they were hiding in plain sight. They were so big–about a hundred times bigger than typical viruses–that scientists mistook them for bacteria. But a close look revealed that they infected amoebae and built new copies of themselves, as all viruses do. And yet, as I point out in A Planet of Viruses, giant viruses certainly straddle the boundary between viruses and cellular life. Flu viruses may only have ten genes, but giant viruses may have 1,000 or more. When giant viruses invade a host cell, they don’t burst open like other viruses, so that their genes and proteins can disperse to do their different jobs. Instead, they assemble into a “virus factory” that sucks in building blocks and spits out large pieces of future giant viruses. Giant viruses even get infected with their own viruses. People often ask me if I think viruses are alive. If giant viruses aren’t alive, they sure are close.
Ever since giant viruses were first unveiled seven years ago, scientists have argued about the origins of these not-so-wee beasties. Many of their genes are different from those found in cellular life forms, or even other viruses. It’s possible that giant viruses amassed their enormous genetic armamentarium over billions of years, picking up genes from long-extinct host or swapping them with other viruses we have yet to find. Other scientists have suggested that giant viruses started out giant–or even bigger than they are today. Some have even argued that they represent a new domain of life, although others aren’t so sure.
More here.