During the past two decades, virus taxonomy has advanced to the point where most viruses can be classified as belonging to families, genera, or groups of related viruses. Virus classification is primarily based on chem- ical and physical similarities, such as the size and shape of the virion, the nature of the genomic nucleic acid, the number and function of com- ponent proteins, the presence of lipids and of additional structural fea- tures, such...