Viruses are not usually consider alive - they are best regarded as delinquent genes, roaming the streets and causing trouble as they break into cells and take over. They can't do any biochemistry for themselves, and hijack the host cell's metabolism instead. Some trigger cancer when they plonk themselves down into important oncogenes. Others save the host cell, by installing New Improved UV-Resistant Chlorophyll into a dying blue-green algae, but run the rest of the as a viral factory in return. A virus may even be the source of the vertebrate immune system, hundreds of millions of years ago.
In 1992 some microbiologists poking around a cooling tower in Bradford found something they thought was a bacterium, parasitising the freshwater protist Acanthamoeba ( Acanthamoeba can live in soil, water, under contact lenses, or in your brain. The later two are very bad news ). Naturally, they were intrigued - for one thing another bacterium - Methicillin-resistant Staphylococcus aureus (MRSA) - can breed inside Acanthamoeba too, and make a lot of people very unhappy or dead.
But when they tried to amplify it's 16S ribosomal RNA, they didn't get a damned thing, which baffled them, so they went back for closer look, and in 2003 Didier Raoult and his colleagues at the Universitee de la Mediterranee discovered that the 'Bradford coccus' was actually a virus - a virus so huge that it was visible under an optical microscope, so gigantic it's bigger than some bacteria, and a genome so humongous that a number of bacterial species look positively puny - it even has, amongst its 911 known genes, instructions for nucleotide and amino acid synthesis that some unarguably alive things lack!
There's some evidence Mimivirus can infect humans too, causing viral pneumonia.
And this year Raoult's team went to a cooling tower in Paris and found one even bigger.
They're calling it Mamavirus, presumably because "Holy Shit, That's One Big Mutha"
And it gets even more interesting - because they noticed Mamavirus was frequently associated with a smaller virus they nicknamed Sputnik. But when they tried to culture Sputnik by itself, in a culture of yet more amoeba, they couldn't get it to reproduce. Sputnik would only replicate in a cell already infected with Mamavirus, or other mimiviruses. But when it does, the mimiviruses go completely to hell - 70% reduction in production, and all sorts of bizarre mutations.
Other viruses dependent on the presence of another virus are known - Adeno-associated virus (AAV) ( used heavily in gene therapy ) can only replicate if an adenovirus has inserted it's own replication instructions first, - but Sputnik with its mere 21 genes is sabotaging another virus's factory - the first known case of a viral hyperparasite, and something they've decided to call a virophage - an entirely new class of organism!