Mimivirus

Mimivirus is an extra-large virus. It was discovered in 1992.
APMV was found accidentally inside the amoeba "Acanthamoeba polyphaga". The virus was seen in a gram stain and mistakenly thought to be a gram-positive bacterium.
It is either a viral genus with a single species, Acanthamoeba polyphaga mimivirus (APMV), or it is a group of phylogenetically-related large viruses (MimiN). 
In speech, APMV is usually referred to as "mimivirus". Until October 2011, when an even larger virus "Megavirus chilensis" was described, it was the largest (capsid diameter) of all known viruses. 
Mimivirus has a large and complex genome compared with most other viruses. Mimivirus, short for "mimicking microbe", is so called for its large size and Gram-staining properties.