From American Scientist:
The common view of viruses, mostly true, is of tiny burglars that sneak into cells, grab the biosynthetic controls and compel the cell to make huge numbers of progeny that break out of the cell and keep the replication cycle going. Viruses are supposed to be diminutive even compared to cells that are just a micrometer (1,000 nanometers) in diameter. They are supposed to travel light, making do with just a few well-adapted genes.
In 1992, a new microorganism was isolated from a power-plant cooling tower in Bradford, England, where Timothy Robotham, a microbiologist at Leeds Public Health Laboratory, was seeking the causative agent of a local pneumonia outbreak. His search led to the warm waters of the cooling tower, a known reservoir for bacterial pathogens in the Legionella genus, which are the cause of the pneumonialike Legionnaire’s disease. Particles present in the sample were mistakenly identified as bacteria. Gram positive and visible under the microscope as pathogens within the particle-gobbling amoeba Acanthamoeba polyphaga, the entities surprisingly did not generate any product from the gene-amplifying polymerase chain reaction technique using universal bacterial primers.
Eleven years later, in 2003, the mystery organism received a new identity and a new name, Acanthamoeba polyphaga Mimivirus, for microbe-mimicking virus. Mimivirus is the largest virus ever discovered. Giant viruses had been known for a few years, many of them in a group termed nucleo-cytoplasmic large DNA viruses (NCLDVs). This group features several other virus families, including Poxviridae, which infects vertebrates (for example, smallpox virus) and invertebrates, the aquatic viruses Iridoviridae and Phycodnaviridae, and the vertebrate virus Asfarviridae. Giant viruses are considered to be ones with genomes larger than 300 kilobase pairs and with capsid diameters of about 200 nanometers or more.
Mimivirus is a giant among giant viruses, with a diameter of 750 nanometers. It possesses a genome, truly outsized by viral standards, of 1.2 million base pairs, coding an outlandish 1,018 genes. For comparison, the smallest free-living bacterium, Mycoplasma genitalium, is just 450 nanometers in diameter and possesses a genome half the size of that in mimivirus, while coding just 482 proteins.