Chapter 20

Viruses

Viruses have become a very topical subject thanks to the Covid-19 pandemic, which

has focused attention onto these fascinating biological objects. ¹ In the past, viruses

were discovered and investigated in the context of a host, and knowledge about them

was relatively limited, encompassing only a few hundred different viruses. The great

enhancement of DNA and RNA sequencing technology, paralleled by similarly great

advances in data analysis, and the advent of metagenomics (Sect. 17.8), enabling

viruses to be discovered without reference to a host, have led to a vast increase in

the number of viruses investigated, and an appreciation of their enormous genomic

diversity, as well as their enormous numbers (perhaps in excess of 10 Superscript 3010³⁰ individuals

on Earth). Their origin is still being debated. ² Despite their structural simplicity, it

is not currently thought that they are very ancient; they may well have first emerged

after the Cambrian period. Great sophistication is, in fact, needed to create a minimal

replicating entity that absolutely requires hosts, which must, therefore, already have

been in existence; the molecular machinery of the host is recruited to serve the ends

of the virus. The smaller viruses are below 100 nm in diameter and are, therefore,

true nano-objects.

Realization of the incredible diversity of viruses is now manifesting itself in the

expansion of the resolution of viral taxonomy, ³ which now has 15 ranks: realm, sub-

realm, kingdom, subkingdom, phylum, subphylum, class, subclass, order, suborder,

family, subfamily, genus, subgenus, and species (cf. Table 5.1). The genetic material

can be RNA or DNA, single-stranded or double-stranded.

1 Frishman and Marz (2002).

2 Cf. Campell (2001).

3 Overseen by the International Committee for Taxonomy of Viruses.

© Springer Nature Switzerland AG 2023

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