Viralogy: a guide to the basics

Viruses are ultramicroscopic, metabolically inert infectious agents that replicate only within living cells. Viruses occur universally and affect every animal, plant and eukaryotic micro-organism on the planet and, although some viruses affect the health of their hosts, most have very little or no impact. Unlike every other domain of life, which has double-stranded DNA (dsDNA) as its genetic program, virus genomes may be composed of either DNA or RNA. The genetic material is then arranged as either double or single strands. Single-stranded RNA (ssRNA) viruses are then further classified as being either positive-sense or negative-sense with respect to the messenger-RNA coding strand. The virus genome can have a linear or circular arrangement and consist of a single or multiple segments.