Viruses are classified through several taxonomic systems based on their characteristics such as nucleic acid composition, structure, host organism, and morphology. The International Committee on Taxonomy of Viruses is responsible for the formal classification of viruses. Key classification systems include grouping viruses by morphology, chemical composition, replication method (Baltimore classification), symmetry, nucleic acids, hosts, and whether they infect animals, plants, or bacteria.

2. TAXONOMY OF VIRUS
 Virus classification is the process of naming viruses
and placing them into a taxonomic system similar to
the classification system used for cellular organisms.
 Viruses are classified by phenotypic characteristics,
such as nucleic acid, structure, host and morphology.
 The formal taxonomic classification of viruses is the
responsibility of the International Committee on
Taxonomy of Viruses (ICTV) system.

3.  Viruses are classified on the basis of morphology, chemical
composition, and mode of replication.
 Th. e viruses that infect humans are currently grouped into
21 families, reflecting only a small part of the spectrum of
the multitude of different viruses whose host ranges extend
from vertebrates to protozoa and from plants and fungi to
bacteria.
The following properties have been used as a basis for
the classification of viruses.
 Virion morphology, including size, shape, type of
symmetry, presence or absence of peplomers, and presence
. or absence of membranes.

4. BALTIMORE CLASSIFICATION
 Baltimore classification (first defined in 1971) is a classification
system that places viruses into one of seven groups depending on
a combination of their nucleic acid (DNA or RNA), strandedness
(single-stranded or double-stranded), sense, and method
of replication.
 Named after David Baltimore, a Nobel prize-winning biologist,
these groups are designated by Roman numerals . Other
classifications are determined by the disease caused by the virus
or its morphology, neither of which are satisfactory due to
different viruses either causing the same disease or looking very
similar.
 In addition, viral structures are often difficult to determine
under the microscope. Classifying viruses according to
their genome means that those in a given category will all behave
in a similar fashion, offering some indication of how to proceed
with further research. Viruses can be placed in one of the seven
following groups :

5.  I: dsDNA
viruses (e.g. Adenoviruses, Herpesviruses, Poxviruses)
 II: ssDNA viruses (+ strand or "sense") DNA
(e.g. Parvoviruses)
 III: dsRNA viruses (e.g. Reoviruses)
 IV:(+)ssRNA viruses (+ strand or sense) RNA
(e.g. Coronaviruses, Picornaviruses, Togaviruses)
 V: (−)ssRNA viruses (− strand or antisense) RNA
(e.g. Orthomyxoviruses, Rhabdoviruses)
 VI: ssRNA-RT viruses (+ strand or sense) RNA with DNA
intermediate in life-cycle (e.g. Retroviruses)
 VII: dsDNA-RT viruses DNA with RNA intermediate in life-
cycle (e.g. Hepadnaviruses)


6. BASED ON STRUCTURE
1. Cubical virus: They are also known as icosahedral
symmetry virus. Eg. Reo virus, Picorna virus.
2. Spiral virus: They are also known as helical symmetry
virus. Eg. Paramyxovirus, orthomyxovirus.
3. Radial symmetry virus: eg. Bacteriophage.
4. Complex virus: eg. Pox virus.

8. BASED ON HOST
The virus can be classified on the basis of the type of
host. They are:
1. Animal viruses
2. Plant viruses
3. Bacteriophage

9. ANIMAL VIRUS :
 The viruses which infect and live inside the animal cell
including man are called animal viruses. Eg; influenza
virus, rabies virus, mumps virus, poliovirus etc. Their
genetic material is RNA or DNA.
PLANT VIRUS :
 The viruses that infect plants are called plant viruses.
Their genetic material is RNA which remains enclosed in
the protein coat. Some plant viruses are tobacco mosaic
virus, potato virus, beet yellow virus and turnip yellow
virus etc.

10. BACTERIOPHAGES :
 Viruses which infect bacterial cells are known as
bacteriophage or bacteria eaters. They contain DNA as
genetic material. There are many varieties of
bacteriophages. Usually, each kind of bacteriophage will
attack only one species or only one strain of bacteria.