Viruses are obligate genetic parasites that require the replication machinery of a cellular host in order to complete their own reproductive cycles. Outside of host cells, viruses exist as particles consisting of a nucleic acid genome surrounded by a protein coat, known as a capsid. Depending upon the specic virus, the genome may comprise DNA or RNA, which in turn may be double or single stranded. Viral capsids may be icosahedral, spherical, pleomorphic, lamentous, or rod shaped. Virus structure may also include additional features such as lipid envelopes, apical spikes, and tails (Figure 24.1). Soils provide an incredible range of niches housing a diverse
array of organisms, leading to a similarly diverse representation of viruses that parasitize these organisms. Soils have been shown to contain plant viruses (Fillhart et al., 1998; Delogu et al., 2003), insect viruses (Fuxa, 2004; Christian et al., 2006), fungal viruses (Melzer and Bidochka, 1998), animal viruses (Duboise et al., 1979; Santamaria and Toranzos, 2003; Pourcher et al., 2007), and bacterial viruses (also known as bacteriophages or simply “phages”) (Yin et al., 1997; Ashelford et al., 1999a, 2000, 2003; Keel et al., 2002; Williamson et al., 2003, 2005, 2007).