Viruses are obligate parasites. The field of study in which viruses are investigated is called virology. Viruses are the agents considered on the borderline of living organisms, come in various shapes, and are widely distributed in nature, infecting animal cells, plant cells, and microorganisms also.

Once a virus infects a host cell it can direct the cell machinery to produce more viruses.

Viruses are made up of genetic material, either DNA or RNA, may be single stranded or double stranded, which is surrounded by protein coat called capsid. It is a protective layer made up of proteins and glycoprotein. Sometimes the capsid is surrounded by an additional spiky envelope that allows viruses to penetrate host cells through membrane fusion.

The entire infectious virus particle is called a virion, which is consists of the nucleic acid and an outer shell of protein.

The simplest viruses contain only enough RNA or DNA to encode four proteins. The most complex can encode 100 – 200 proteins.

Viruses are obligate parasites because they lack metabolic machinery that can produce energy. They cannot synthesize protein so they are dependent on host cells for these basic vital functions. Once inside a cell, viruses have genes for seizing energy producing and protein synthesizing systems of cell.

Since many viruses can infect a large number of different cell types, genetically modified viruses often are used to carry foreign DNA into a host cell. This approach provides the basis for a growing list of experimental gene therapy treatments.



All viruses have six basic steps in their replication cycle:

1) Attachment

2) Penetration

3) Uncoiling

4) Replication

5) Assembly

6) Release


The lytic cycle of the virus can be explained as:

1) The virus first attaches itself to the host cell, which is accomplished through special glycoprotiens that are present on the exterior of the capsid, envelope or tail.

2) Now, penetration occurs, either the whole virus or just the contents of the capsid is penetrated in to the host cell.

3) If the entire capsid enters, the genetic material is first uncoated to make it available to the host cell’s replication machinery.

4) Replication of genetic material now takes place.

5) Once all of the necessary parts have been replicated, individual virus particles are assembled inside the host cell only and after the assembly is completed the viruses are then released out of the host cell.

6) Release often takes place in a destructive manner i.e. by bursting or killing the host cell.

In the lysogenic lifecycle of the virus, the viral genetic material that has entered the host cell becomes incorportated in the cell and lies dormant. It is passed on to the progeny of the infected cells. Eventually, the lytic phase will start again, and cells that were never infected themselves, but carry the viral genetic material will begin to produce new virus particles.



Viruses are inert outside the host cell. Some viruses like polio, can even be crystallized and still be viable. The main purpose of a virus is to deliver its genome into the host cell to allow its expression (transcription and translation) by the host cell.



  1. One of the most important roles of virus structure is a protective role. The capsid structure protects the viral genome from physicochemical damage, such as nucleases, and radiation (eg, ultraviolet).
  2. The role of the virus structure is to recognize the cellular receptor for the entry. Specifically, one of the viral structural proteins (either an envelope glycoprotein for the enveloped viruses or a capsid protein for the nonenveloped viruses) directly binds to the cellular receptor, for the viral entry.
  3. The viral capsids play a role in delivering the viral genome to the site of genome replication. For instance, for the viruses that replicate in the nucleus, the viral capsids play a critical role in the nuclear entry of the viral genomes


The simplest virion consist of two basic components:

  • Nucleic acid (single- or double-stranded RNA or DNA)
  • Capsid: A protein coat. It functions as a shell to protect the viral genome from nucleases and which during infection attaches the virion specific receptors that are present on the host cell.
  • They sometimes also contain an envelope. They consist of a lipid bilayer that closely surrounds a shell of virus-encoded membrane-associated proteins.



Lwoff, Horne, and Tournier suggested a comprehensive scheme for classifying all viruses in 1962. Their proposal used the classical Linnaean hierarchical system of phylum, class, order, family, genus and species.

Viral morphology provides the basis for grouping viruses into families.

A virus family may consist of members that replicate only in invertebrates, only in vertebrates, only in plants, or only in bacteria. Some may replicate in more than one of these hosts.

Most of the vertebrate viruses have been classified or named according to:

  • the diseases associated with them(poliovirus, rabies)
  • the type of disease caused by them (murine leukemia virus),
  • the sites in the body they affect or from in which the virus was first isolated (rhinovirus, adenovirus)
  • the places from where they were first isolated (Sendai virus, Coxsackievirus)
  • the scientists who discovered them (Epstein-Barr virus), or
  • common cultural perceptions e.g. influenza ‘influence’ of bad air or dengue ‘evil spirit’


ICTV classification

Naming convention of the viruses primarily depends on the genome and nucleic acid material of the viruses. Naming is performed by the International Committee on the Taxonomy of Viruses (ICTV). A complete catalogue of known viruses is maintained by the ICTV at ICTV database.

The order of the classification of the viruses is as follows:

  • Order – virales
  • Family –viridae
  • Subfamily –virinae
  • Genus –virus
  • Species –virus


In the 2011 ICTV classification, there are six orders:

  • Caudovirales
  • Herpoesvirales
  • Mononegavirales
  • Nidovirales
  • Picornavirales
  • Tymovirales

The seventh i.e. Ligamenvirales has been proposed.


The Baltimore classification

The name comes from the Nobel Prize winner David Baltimore. This classifies according to the viral mRNA synthesis.

At present both ICTV and Baltimore classification are used together, for instance:

  • Group I possesses double stranded DNA
  • Group II posses single stranded DNA
  • Group III has double stranded RNA
  • Group IV has positive single stranded RNA
  • Group V has negative sense single stranded RNA.
  • Group VI further has single stranded RNA with reverse transcriptase that converts RNA to DNA like HIV virus.
  • Group VII has double stranded DNA with reverse transcriptase and this includes Hepatitis B virus.