The size of a small bacterium is very large for a virus. The viruses are five times smaller. However, it is one of these dimensions, discovered by French researchers in 2003 and called Mimivirus.
Usually viruses are not considered alive, since they are not able to reproduce themselves, but have to infect other cells to reproduce with their genes. But this mimivvirus has more genes than conventional viruses, including some essential to repair DNA and synthesize proteins. Although these additional genes do not confer full reproduction capacity on their own, French researchers believe that Mimivirus can be considered a living.
Furthermore, the virus is more or less capable of doing what the nucleus of a cell does. Therefore, they have launched a new hypothesis to explain the genesis of the nucleated cell, which is the result of the introduction of this Mimivvirus by cells without nucleus. It is not the only hypothesis, but the first one to consider a virus.