Mimivirus is the largest known virus. Specifically, Mimivirus refers to a viral genus, but it only contains one species - Acanthamoeba polyphaga mimivirus (APMV), so the terms are used interchangeably. The Mimivirus was discovered by accident in 1992 during research into Legionnaires’ disease being conducted by French scientists in Marseille. It was not until 2003 that a paper was published identifying the organism as a virus and explaining some of its workings. The reaction from the microbial research community since then has been substantial. Some even go so far as to call Mimivirus a "missing link" between viruses and bacteria.
Mimivirus is one of several known DNA viruses, relatively rare and large in comparison to their cousin the RNA viruses. Herpes simplex virus, the virus responsible for Type 1 herpes, is another example of a DNA virus. The Mimivirus is so large (400 nm) that scientists originally thought it was a gram-positive bacterium when they first discovered it. The virus is well-known because, being the largest virus yet observed, and due to other qualities, it blurs the line between viruses and bacteria. The Mimivirus genome consists of 1.2 million base pairs. Based on metagenomic data, it is suspected that other viruses in the same family - Mimiviridae - probably exist. Searches for them are currently underway, and new microbes are being discovered all the time.
The Mimivirus infects amoebas (where it was first discovered), and poses no threat to human beings. Its structure is that of a polygon covered with fibrils (hairs) that, when included in the measurement of the virus, can double its apparent size. Like other large viruses, it has an internal lipid layer that protects its DNA core. Its genome is roughly 1.2 million base pairs in length, and has not yet been sequenced. This genome is twice the size of the next largest known viral genome. Although, like all viruses, it does not have a cell wall, Mimivirus is larger than at least 30 of the smallest known cellular organisms. The Mimivirus genome has unique genes that code for metabolic functions not seen in any other viruses, and as such its study will teach us important new things about life in its most basic forms.
Mimivirus is so unusual that some scientists have suggested that it may be part of a new kingdom of life. But since the life classification system has only recently been remodeled, and most scientists would not consider creating a new kingdom of life with only one member, no matter how different it is, this is not likely to be accepted by the wider biological community.