ABSTRACT

Viruses are submicroscopic infectious agents that are usually made up of a nucleic acid core (DNA or RNA) with a protective protein coat called capsid. The capsid may appear helical or icosahedral (polyhedral or near spherical). Some viruses have more complex structures with tails or an envelope that is derived from the host cellular membrane. Based on the type of nucleic acids they contain, viruses are classifi ed into two major groups: DNA viruses that possess deoxyribonucleic acid (Chapter 2), and RNA viruses that have ribonucleic acid as their genetic materials.