Virus are small structure which can pass its genetic material to living host where it can replicate. They can infect plants, animals and other microorganisms also. The study of viruses is called virology which is considered as a subspecialty of microbiology.  Martinus Beijerinck is the father of virology.

In 1898, Friedrich Loeffer and Paul Frosch was the first to find evidence of the nature of viruses, genetic entities of virus. They found the evidence from foot and mouth disease in livestock. The first identified human virus was yellow fever caused by mosquito Aedes aegyptii.Virus was discovered and reported in 1910 by a US Army physician.Veterniary virology gained importance after in1902 after cattle plague.


Virus classification is the process of naming virus and places them into a taxonomic system. Virus are mainly classified by phenotypic characters like morphology, mode of replication, host organisms, nucleic acid type and the type of disease caused by the virus. A universal system for classifying viruses, and a unified taxonomy was established in1966 by International Committee on Taxonomy of Viruses (ICTV).vanRegenmortel (19) lists the following characters for discriminating between virus species:

  • Relatedness of genome sequence
  • Natural host range
  • Cell and tissue tropism
  • Pathogenicity and cytopathology
  • Mode of transmission
  • Physicochemical properties of viral proteins
  • Antigenic properties of viral proteins.

The system makes use of a series of ranked taxons,with the: order,family,subfamily,genus,species,order,family,genus,species.


It is classified based on shape and size. Based on the structure of nucleocapsid it’s divided into two namely helical morphology and icosahedral morphology. Helical morphology consist of a helical array of capsid proteins which is surrounded by a helical filament of nucleic acid. The number and arrangement of the capsomers are useful in identification and classification of icosahedral morphology Based on envelope it’s divided as enveloped and non-enveloped virus.


  • It possess no membranes,cytoplasm,ribosomes or other cellular components
  • They cannot move or grow
  • They are really tiny, even smaller than cell and is only visible advanced electron microscopes.
  • It consist of two major parts which are a protein coat and a hereditary material which can be a RNA or DNA.


Viruses cannot replicate on its own and in order get multiplied it must infect a host. It replicates in host cell’s machinery to create more viruse.steps involved in replication includes:

  • Penetration: The virus gets engulfed by the cell or get attached to some other viruses which have surface proteins that bind to receptors on the host cell. It allows the host cell to either fuse with virus or absorb the virus. Once it reaches the cell, the genetic material is released to the cytoplasm.
  • Replication: The genetic material of the virus is copied several times.
  • Transcription: The genetic material act as a blue print for the cell to make messenger RNA which is used to make viral proteins.
  • Protein Assembly: It occurs in the cytoplasm (ribosome) where viral proteins are made.
  • Viral Assembly: The viral genetic material (after replication) will get surrounded by the newly made viral protein.
  • Release: Through budding viruses get emerged from the cell through cell membrane or by bursting out of the cell which will result in the death of host cell.


Size of a virus is smaller than bacteria and size is variable. The larger viruses can vary their size about 300 A in diameters. It indicates that it may be as larger as a small bacteria. They occur mainly in three shapes which are helical (spherical or can be of complex symmetry. Virus lacks cytoplasm and hence lack cell organelles like mitochondria, Golgi bodies, ribosomes and enzyme systems are also present. Virus contains RNA or DNA whereas a normal cell contains both DNA and RNA. It contains a protein coating called capsid which acts as a protection for genetic materials by surrounding the genetic material. A virus particle is called virion.

Plant virus contains only RNA.Animal virus contains both RNA or DNA.wheras bacteriophages contain only DNA.


  • Double-Stranded DNA: Double-Stranded DNA virus generally have a polyhedral or complex structure. Examples are Papilloma Virus,Variola(smallpox)
  • Double-Stranded RNA: Double-Stranded RNA usually have polyhedral structure. Example is diarrhea virus.
  • Single –Stranded RNA: Single –Stranded RNA virus have two subunits which can serve as mRNA and other one which can serve as a template mRNA.Examples are Rhinovirus,HIV


The tumor viruses change cells by integrating their genetic material with the host cell DNA.This is a permanent insertion in the genetic material which is never removed. The insertion mechanism can differ depending on whether the genetic material of the tumor causing DNA or RNA.In case if the genetic material is DNA then it can be directly inserted into the host DNA.In case of RNA as the genetic material, then RNA is first transcribed into DNA and then it is inserted into the host cell’s DNA.



Structure, Characteristics and classification


  • Virus is a parasite of sub-microscopic level on all the organisms. They infect all type of life forms like plants, animals, bacteria.
  • They are ambiguous in nature i.e. weather living or non-living, they are non-living in free-state but behave like living organism in a host.
  • In 1892  Dmitri Ivanovsky’s described about virus when he was working on bacterial pathogen infecting tobacco plants.
  • When virus invades a host organism it forced to replicate rapidly to produce thousands of copies of the virus. In contrast when virus is not inside a host or in the process of infecting it, virus exists in independent form known as ‘virions’ consists of a genetic material (RNA or DNA), a protein coat – capsid and the outer envelope of lipids.
  • Virus can be of various shapes ranging from helical to the icosahedral form.
  • Virus are much smaller than bacteria, size of the most virus that have been studied is between 20-300 nm.
  • They contain only one type of nucleic acid at a time either DNA or RNA but never both.
  • Virus are responsible for various infectious diseases in plant and animals rabies, AIDS (HIV), avian influenza, Ebola virus disease. Transmission of viral disease is through ‘vector’ (diseases bearing organism).


Virus shows a wide variety of shape and size, a complete virus particle know as virions contain nucleic acids surrounded by the protein coat capsid, capsid is made up of small subunits of protein knows as capsomers and an outer layer called envelope made up of lipid which is derived from host cell membrane.

The main morphological structures are:


  • Composed of single type of capsomeres stacked around the central axis of helical structure having central cavity results in the formation of rod shape structure which may be short and rigid or long and flexible
  • Genetic material (typically ssRNA or ssDNA) bound to the protein helix by the interaction between the negatively charged nucleic acid and the positively charged capsomer protein.

Example: tobacco mosaic virus


  • Most animal viruses exhibit icosahedral symmetry. The minimum no of triangular faces is three which gives rise to the 60 more capsomers. Rotavirus has more than 60 capsomers. Regular icosahedron is the optimum way to form closed shell symmetry with identical subunits.
  • Capsomers attaches with 5 other capsomers at the apices called pentons and on the triangular faces attaches with 6 capsomers called hexon.
  • Pentons and hexon may be of same protein or may be of other proteins.


  • Elongation of icosahedron along the fivefold axis, it is common in bacteriophage and composed of a cap with cylinder structure.


  • In some species, the virus modify their cell membranes to envelope them self it may be either the outer membrane of the host or the internal nuclear membrane or endoplasmic reticulum making a outer lipid bilayer known as viral envelope.
  • The infectivity are depend on the envelope of the virus, the membrane i
  • Has protein coded by the viral and host genome and lipid membrane with any carbohydrates originate by the host.


  • Non- cellular organism enclosed by a protective envelope
  • Virus attaches to their host by the help of spikes
  • Having nucleic acid (DNA and RNA) in the core which is surrounded by the protein coat
  • Considered as both living and non-living i.e. inactive when present outside the host and became active within the host cells
  • Virus uses host mechanism and enzymes to reproduce itself.

Classification :

On the basis of genetic material:

DNA virus:

  • DNA as genetic material
  • They attack on both humans and animals
  • Example: papillomavirus, parovirus and herpesvirus

RNA virus:

  • RNA as genetic material
  • Example: polio virus, ebola virus, hepatitis C virus

DNA-RNA viruses:

  • Having both DNA and RNA as genetic material
  • Example: leukoviruses, rous’s viruses

On the presence of number of strands:

Double-stranded DNAherpes viruses, adenoviruses
Single-stranded DNAbacteriophagesφ, X, 74 bacteriophages
Single-stranded RNAinfluenza virus, poliomyelitis, bacteriophage MS-2
Double-stranded RNArice dwarf viruses, wound tumour virus

On the basis of envelope:


DNA viruses:  poxviruses, herpesviruses

RNA viruses: toga virus, coronavirus


DNA viruses: adenovirus, papovirus

RNA viruses: hepatitis A and E virus

On the basis of capsid structure:

Naked icosahedralPoliovirus, hepatitis A virus
Enveloped icosahedralRubella virus, HIV-1
Naked helicalTMV
Enveloped helicalMumps virus, measles virus
ComplexSmallpox virus, hepatitis B virus

On the basis of shape:

Space vehicle shapedAdenovirus
Filamentous shapedEbola virus
Brick shapedPoxvirus
Bullet shapedRabies virus

On the basis of type of host:

Animal viruses:

  • Viruses infect and live inside the animal cell
  • Example: rabies virus, mumps virus

Plant viruses:

  • Their genetic material is RNA remains enclosed in the protein coat and infect plants
  • Example: potato virus, TMV, turnip yellow viruses


  • Virus infect bacteria are known as bacteriophage, contain DNA as genetic material

On the basis of mode of transmission:

Transmitted throughExample
Respiratory routeRhino virus, swine flu
Faeco-oral routePolio virus, rota virus
Blood transfusionHIV, hepatitis B virus
Sexual contactRetro virus
Zoonotic virusAlpha virus, flavi virus

Baltimore Classification:

  • In early 1970s Nobel prize winner David Baltimore developed the most commonly used virus classification system
  • Baltimore focus on how mRNA is produced during replication and classify virus into various groups

Group I: mRNA is produced as the same way as their cellular DNA by transcription, contain ssDNA as their genetic material

Group II: convert their ss genome dsDNA before transcription of mRNA occur, have ssDNA as genetic material

Group III: uses RNA dependent RNA polymerase to generate mRNA from the one of the strand used as template, genome is dsRNA.

Group IV: genetic material is ssRNA . Genomic RNA is with positive polarity means that is directly serve as mRNA

Multiple full length RNA strand with negative polarity are formed from the intermediate of dsRNA (replicative intermediates) made in the genomic copying process.

These serve as template form the production of positive polarity RNA

Group V: contain ssRNA with negative polarity; means that sequence is complementary to mRNA. Negative-strand is converted into mRna

Group VI: virus have two copies of genome i.e. ssRNA  conveted using reverse transcriptase enzyme to dsDNA

dsDNA transported to the host’s nucleus in inserted in its genome viral DNA is produced by transcription with host DNA

group VII: genetic material is dsDNA which make ssRNA as intermediate acts as mRNA and also converted back to dsDNA  by reverse transcriptase enzyme.