TO THE PRINCIPLES
Peter Medawar, a biologist awarded the Nobel Prize for Medicine and Physiology in 1960, defined viruses as a piece of nucleic acid surrounded by bad news ( 1). True, viruses are nothing more than a speck of genetic material -- a single kind of nucleic acid (segmented or nonsegmented, DNA or RNA) and a coat made of protein molecules. Viruses multiply according to the information contained in this nucleic acid. Everything other than the DNA or RNA is dispensable and serves primarily to ensure that the viral nucleic acid gets to the right part of the right sort of cell in the organism hosting the virus, because viruses cannot multiply until they invade a living cell. Viruses enter all cellular forms of life from plants and animals to bacteria, fungi, and protozoa. Viruses, plants, and animals form the three main groups that encompass all living things. Plants and animals are cellular organisms and include bacteria and protozoa. Viruses lack cell walls, are obligatory parasites, and depend for replication on the cells they infect.