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Small viruses are predominant and may be round, or rod shaped, or a combination of the two in the case of prokaryotes. A capsid is the protein shell of a virus, which can remind us of the idea of Platonic solids. For some small viruses, usually the self-assembly process is dictated by their nucleic acids sequence (motif seeds) [90]. This protein shell seals their genetic material from the environment. However, many capsid proteins can self-assemble with no additional help. Viral proteins have structural properties that allow regular and repetitive interactions among them. Identical protein subunits are distributed with helical symmetry for rod-shaped viruses and polyhedron symmetry for round viruses. Because they have small genomes, viral genes must repeat protein subunits. Each subunit has identical bonding contacts with the neighbors. Repeated interaction with chemically complementary surfaces at the subunit interfaces, naturally leads to a symmetric arrangement (3D patterns). The bonding contacts are usually noncovalent. This ensures error-free self-assembly and reversibility. Thus, if the gene responsible for the viral protein is inserted into another cell for expression, that cell will produce viral proteins that will self-assemble into shells – fake viruses with no genome inside. Depending on the species, the 3D shape and the repetitive interactions of viral proteins allow for different types of bonding patterns, which in turn lead to different configurations and capsid sizes (Table 1.3.).

Thus, some viruses can be comparable in size with certain life forms (Table 1.4.). For a degree of comparison, M. gallicepticum is 3 up to 10 times smaller than the diameter of the largest giant viruses. Giant viruses that infect single-celled eukaryotes like amoebas (i.e. Acanthamoeba castellanii), such as Pithovirus sibericum, Pandoravirus salinus, or Pandoravirus dulcis, are about 1–1.5 μm (1000–1500 nm) in length [91, 92]. Other more well-known giant viruses are Megavirus chilensis (400 nm) or Acanthamoeba polyphaga Mimivirus (390 nm), each with considerable dimensions over the size of certain prokaryotes [91]. In terms of physical size and genome complexity, giant viruses closed a significant gap between the realms of viruses and the prokaryotic/eukaryotic unicellular organisms [91].

Table 1.3 Extreme sizes in viruses.

Viruses	Eukaryotic viruses (μm)	Prokaryotic viruses (μm)
Min	0.017	0.03
Max	1.5	0.2

The table shows the minimum and maximum physical dimensions of viruses found in both eukaryotes and prokaryotes. The values represent averages of the measurements published in the scientific literature and are presented in micrometers.

Table 1.4 Single-celled organisms vs. viruses.

	Eukaryotes	Prokaryotes	Eukaryotic viruses	Prokaryotic viruses
	Species	Val (μm)	Species	Val (μm)	Species	Val (μm)	Species	Val (μm)
Min	Prasinophyte algae	0.8	Mycoplasma genitalium	0.15	Porcine circovirus	0.017	Phages	0.03
Max	Caulerpa taxifolia	300 000	Thiomargarita namibiensis	1400	Pithovirus sibericum	1.5	Phages	0.2

The table shows a comparison between extreme microscopic sizes of viruses and unicellular organisms, that covers both eukaryotes and prokaryotes.

On the other scale, Porcine circovirus is the smallest virus (17 nm) and is found in multicellular eukaryotes [93, 94]. Almost all isolated viruses from prokaryotes show ranges between 30 and 60 nm. Giant prokaryotic viruses with capsids diameters ranging from 200 to more than 700 nm have been reported [95]. Nevertheless, these comparisons between virus sizes in prokaryotes and eukaryotes can be misleading as more specialized life forms can lead to more extreme variations in size, complexity, and methods of infection.