Principles of Molecular Virology points out that size alone does not distinguish viruses from other microbes such as bacteria. And new virus discoveries just keep getting more diverse.
It's a common mistake that "viruses are smaller than bacteria". While that's true in most cases, it's not always so. The largest viruses have longer genomes and bigger particles than the smallest bacterial cells, so size alone does not distinguish them. These viruses are large enough to be visible under a good light microscope. At first, Mimivirus ruled the roost as the largest known virus. Then along came the Megaviridae which were slightly bigger. Recently, another group of giant viruses have been discovered - the Pandoraviruses. Earlier this year, another type of giant virus, Pithovirus sibericum, was isolated from the Siberian permafrost using Acanthamoeba as bait, causing some discussion that revival of such viruses due to thawing of permafrost either from global warming or industrial exploitation of circumpolar regions might cause future threats to human or animal health (Nature News & Comment: Infectious diseases: Smallpox watch).
All these giant viruses have double stranded DNA genomes over a million base pairs long, nearly 3 Mbp in the case of some of the Pandoraviruses. But their particles are distinct in shape, Mimivirus particles having icosahedral capsids of about 0.5 µm in diameter, while Pandoraviruses have "amphora-shaped" particles from 1–1.2 μm in length.
So what are we to make of all these giant viruses (the "nucleocytoplasmic large DNA viruses" as they are now being called)? Clearly they are not so unusual as we thought 10 years ago when Mimivirus was isolated, and recent discoveries show they are much more diverse than we initially assumed. And they proved we still have a lot to find out about virology.
Pandoraviruses: Amoeba viruses with genomes up to 2.5 Mb reaching that of parasitic eukaryotes. (2013) Science 341(6143): 281–286
Ten years ago, the discovery of Mimivirus, a virus infecting Acanthamoeba, initiated a reappraisal of the upper limits of the viral world, both in terms of particle size (>0.7 micrometers) and genome complexity (>1000 genes), dimensions typical of parasitic bacteria. The diversity of these giant viruses (the Megaviridae) was assessed by sampling a variety of aquatic environments and their associated sediments worldwide. We report the isolation of two giant viruses, one off the coast of central Chile, the other from a freshwater pond near Melbourne (Australia), without morphological or genomic resemblance to any previously defined virus families. Their micrometer-sized ovoid particles contain DNA genomes of at least 2.5 and 1.9 megabases, respectively. These viruses are the first members of the proposed “Pandoravirus” genus, a term reflecting their lack of similarity with previously described microorganisms and the surprises expected from their future study.
Thirty-thousand-year-old distant relative of giant icosahedral DNA viruses with a pandoravirus morphology. (2014) Proceedings of the National Academy of Sciences, 111(11), 4274-4279
Giant DNA viruses are visible under a light microscope and their genomes encode more proteins than some bacteria or intracellular parasitic eukaryotes. There are two very distinct types and infect unicellular protists such as Acanthamoeba. On one hand, Megaviridae possess large pseudoicosahedral capsids enclosing a megabase-sized adenine–thymine-rich genome, and on the other, the recently discovered Pandoraviruses exhibit micron-sized amphora-shaped particles and guanine–cytosine-rich genomes of up to 2.8 Mb. While initiating a survey of the Siberian permafrost, we isolated a third type of giant virus combining the Pandoravirus morphology with a gene content more similar to that of icosahedral DNA viruses. This suggests that pandoravirus-like particles may correspond to an unexplored diversity of unconventional DNA virus families.