Historically, all the viruses were grouped according to the illness they caused (for eg- hepatitis, encephalitis etc.). It was quite common to name the virus on the disease with which it is associated (foot and mouth disease virus) or the geographical location from which it is isolated (Rift valley fever virus). This kind of nomenclature changed with the advent of molecular biology and more advanced biochemical and biophysical techniques.

The most comprehensive and widely used classification was first given by Dr David Baltimore in which seven groups were proposed based on the genetic contents and replication strategies of the viruses.   This classification is focused on the relationship between the viral genome and its mRNA and describes the formation of mRNA by the viruses with either DNA or RNA genome

Figure 15.1. Baltimore classification based on mRNA production by all viruses following infection