The Variety of Life


The word virus is derived from Latin word venome which means poisonous fluid. Virus can be defined as:

Non-cellular, submicroscopic infectious agents which contain either RNA or DNA, enclosed by proteinaceous coat, and reproduce only in living cells (obligate intracellular parasites).

Viruses use biosyntheticmachinery of the host to make their materials and then transfer to other cells. Study of virus is known as virology.


Some viral diseases have been known from centuries.

The first infectious disease against which presentation was developed was a viral disease.

  1. Work of Edward Jenner:

In 1796, Edward Jenner discovered an effective method for the prevention of a viral disease small pox.

He removed material from cowpox lesion on the hand of milkmaid and injected into an 8 years old boy (JamesPhipps). After six weeks the boy was injected with pus from a small pox victim. He did not develop the disease. Jenner used material for vaccination from cowpox lesions and successfully vaccinated 23 persons. As the material was obtained from cow (called vacca in latin), this method was named as vaccination by Louis Pasteur.

  1. Work of Charles Chamberland:

Charles Chamberland (1884) found that bacteria cannot pass through porcelain filters. However agents responsible for rabies can pass through these filters.

Any toxic substance that caused disease was called virus. These unseen filterable agents of rabies were called as filterable viruses.

Rabies is a disease which is transferred to human by bites of rabid dogs, foxes, cats, bats and other animals.

  1. Work of Ivanowski:

In 1892, Ivanowski discovered that the agent which caused tobaccomosaicdisease was filterable.

He obtained bacteria free filtrate from infected plants and placed it on healthy leaves of tobacco.

The filtrate caused the disease in healthy plants.

Later these ultramicroscopic agents were also observed in victims of many diseases including foot and mouth disease (1898) and yellow fever (1901).

In 1898 the Dutchman Beijerink formed the name ‘virus’ (Latin for poison) to describe the infectious nature of certain filtered plant fluids.

Although progress was made in isolating highly purified samples of viruses and in identifying them chemically as nucleoproteins (nucleic acids combined with proteins), the particles still proved mysterious because they were too small to be seen with the light microscope. They were among the first biological structures to be studied when the electron microscope was developed in the 1930s.

Stanley (1935) crystallized the tobacco mosaic virus.


Viruses are small infectious agents and can be seen under electron microscope. They have following characteristics:

  1. Size:

They range in size from 250 nm of Poxviruses to 20 nm of Parvoviruses.

  1. Filterable:

They are 10 to 1000times smaller than bacteria. So they can pass through the pores of

  1. Obligate Intracellular Parasites:

Viruses grow on artificial media. They can reproduce in animal cells, plant cells or in microorganisms.

Here they reproduce by replication (a process by which many copies or replicas of virus are formed). Therefore the viruses are obligate intracellular parasites.

No Metabolic Machinery:

Viruses have no metabolic machinery for the synthesis of their nucleic acid and protein. They depend on the host cell to complete vital functions.

Disease Production:

They can cause disease in the host during reproduction.

Resistant to Antibiotics:

They are generally resistant to many antibiotics such as penicillin, streptomycin and others.

Each type of virus will recognize and infect only certain types of cell. In other words, viruses are highly specific to their hosts.



The complete, mature and infectious particle is known as virion. It has following parts:

  1. Genome:

The virions are composed of a central core of nucleic acid (DNA or RNA) which is also called genome.


The genome is surrounded by a protein coat, the capsid.

Nucleic acid and capsid is collectively called nucleocapsid.

Capsid gives definite shape to virion.

Capsid is made up of protein subunits known as capsomeres.

The number of caspomeres varies in a particular virus.


162 capsomeres in the capsid of herpes virus,

252 capsomeres in the capsid of adenovirus which cause some common colds.

  1. Enveloped or Naked Vireons:
  • In some animal viruses the nucleocapsid is covered by a membrane called envelope. This membrane is derived from the host cell.
  • The viruses which are not enveloped are known as naked vireons.


Animal and plant viruses may be:

  • Polyhedron (having many sides),
  • Helical (spiral),
  • Enveloped or