Discovery and Classification

The virus was first described 120 years ago as a filterable, transmissible agent that causes disease in plants and animals. A virus is a “submicroscopic and intracellular parasite” that can propagate only inside a living cell. The obligatory nature of a virus brought a debate on whether it is living or nonliving. Virology, as a discipline that studies the diverse aspects of viral infection of host cells and its consequence, became established during the early 20th century. Viruses are found in almost all living organisms on earth, ranging from bacteria, fungi, and amoeba, to plants and animals. Viruses are classified into seven groups, according to genomic features. This molecular classification, also called the “Baltimore classification,” allows us to predict precisely the mode of viral genome replication.