Viruses are tiny bits of genetic material, wrapped in protein envelopes, capable of hijacking the cellular machinery of plants, animals, and bacteria for self-replication. Viruses are extremely small, much smaller than bacteria. A typical virus is between 10 and 300 nanometers (billionths of a meter) in size, whereas typical bacteria are larger than a micrometer (millionth of a meter) in size.

Only about 30,000 virus types are being tracked by virologists, although millions likely exist. Viral classification is made difficult by a lack of fossil evidence — viruses fossilize poorly — and controversy over whether viruses are living organisms or not. Unlike other organisms, new viruses sometimes emerge de novo from the genomes of preexisting organisms, making it difficult to build coherent family trees. However, this doesn’t stop virologists from trying.

A unified taxonomy for viruses was only developed in 1966, by the International Committee on Taxonomy of Viruses (ICTV). This classification scheme is inspired by the Linnaean classification system for other organisms. It contains five levels – order, family, subfamily, genus, and species. The recognition of the order level is extremely recent, with only three orders named. Most viral families have not yet been placed in any order. Currently, the ICTV only recognizes about 1,550 viral species.

As an example, the chicken pox virus, Varicella zoster (VZV), belongs to the family Herpesviridae, subfamily Alphaherpesvirinae, genus Varicellovirus, and lacks a categorization based on order.

A second classification scheme, Baltimore Classification, developed by and named after Nobel Prize-winning biologist David Baltimore, is commonly used in conjunction with the ICTV system. The Baltimore classification system categorized viruses based on what the virus is made out of. Here is the scheme:

Group	Contains
I	double-stranded DNA viruses
II	single-stranded DNA viruses
III	double-stranded RNA viruses
IV	(+)single-stranded RNA viruses
V	(-)single-stranded RNA viruses
VI	double-stranded reverse-transcribing viruses
VII	double-stranded reverse-transcribing viruses

By using these two classification systems together, virologists can actually have a clue of what one another are talking about. Another traditional classification system for viruses is based on what type of organism they infect: there are plant-specific viruses, animal-specific viruses, and bacteria-specific viruses, called bacteriophages. Depending on the host the virus has evolved to infect, it will have a different physical structure.