Mimiviruses are the largest viruses so far described (even bigger than some single-cell organisms). They are icosahedral and contain a double stranded DNA genome. The capsid is covered by fibres (fibers) that radiate outwards imparting a bacteria-like outer surface to the organism. In fact it takes up the Gram stain (like Gram positive bacteria) causing early confusion. Mimivirus has been implicated in some cases of pneumonia. Originally found in amoeba.
The size and complexity of mimivirus have provoked questions about the the defining characteristics of viruses and have triggered important questions in evolutionary biology. Viruses have many of the attributes of life. They replicate (with a greater or lesser degree of copying fidelity). They can evolve by natural selection. They are intimately associated with living systems. They are made from standard biological building blocks. Some people regard them as alive whereas others regard them as non-living. They are classified by a standard biological taxonomy. The evolution of viruses is still obscure.