Viruses – An Introduction

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Virus (Latin word for toxin or poison) is a microscopic organism consisting of genetic material (RNA or DNA) surrounded by a protein, lipid (fat), or glycoprotein coat. Viruses are very diverse. They come in different shapes and structures, have different kinds of genomes, and infect different hosts. Influenza, polio, mumps, rabies, small pox, AIDS and dengue are caused by viruses and they cannot be placed in any of the five kingdoms because they are non-living and not organisms. Viruses are very small (smaller than the smallest bacteria) and seen only under electron microscope. Most viruses range in size from 5 to 300 nanometers (1 nanometer = 10-9 meters), although some Paramyxoviruses can be up to 14,000 nm long.

Structure of virus

The virus consists of genetic material, either DNA or RNA, protected by a protein shell called a capsid. Virus can reproduce only when inside living cells. Some viruses also have an outer envelope composed of fatty materials and proteins. The nucleic-acid core is the essential part of the virus; it carries the virus's genes. A number of virus particles are thus formed inside the host cell. The host cell bursts to release the new virus particles.

Virus — living or non-living?

Viruses are inability to exhibit properties of life outside living host cells. Viruses can replicate and multiply inside the host but cannot grow. Protoplasm, cellular organization, cell organelles, metabolic reactions are absent; therefore, viruses are regarded as non-living.

Infective properties of virus

Viruses are attack to bacteria, plants or animals. The viruses that infect bacteria are called bacteriophages; viruses are highly specific in their relationship with the host and tissue. For example – Polio virus attacks particular nerves; mumps virus attacks the particular pair of salivary glands (parotid glands) of humans.

Viruses keep on ‘mutating’!

Mutation mean’s change in genetic material. For example – Flu virus mutates and so every year flu is caused by a different virus and scientists find it difficult to find a cure for influenza or flu.

Viruses and diseases

Viruses show their hosts diseases and modes of their transmission as below table;

Table on viruses and diseases




Mode of Transmission


Potato roll virus


Potato leaf roll

Air borne, contact

Tomato stunt virus


Tomato bushy stunt

Air borne, contact

Tobacco mosaic virus



Air borne, contact


Herpes virus



Air borne, contact

Pox virus


Small Pox

Air borne, contact




i) Sexual contact

ii) Lactating mother to child

iii) Blood transfusion




Bite of infected Aedes mosquito

Hepatitis B



Infected water