Viruses are obligate, intracellular parasites. They cannot reproduce by themselves and must commandeer a host cell’s machinery to synthesize viral proteins and, in some cases, to replicate the viral genome. RNA viruses, which usually replicate in the host-cell cytoplasm, have an RNA genome, and DNA viruses, which commonly replicate in the host-cell nucleus, have a DNA genome (see Figure 5-1). Viral genomes may be single- or double-stranded, depending on the specific type of virus. The entire infectious virus particle, called a virion, consists of nucleic acid and an outer shell of protein that both protects the viral nucleic acid and functions in the process of host-cell infection. The simplest viruses contain only enough RNA or DNA to encode four proteins; the most complex can encode some two hundred proteins. In addition to their obvious importance as causes of disease, viruses are extremely useful as research tools in the study of basic biological processes such as those discussed in this chapter.