Virus - Characteristics, Structure, Classification, Mode of Transmission of Viral Diseases and Control of Viral Diseases

The word virus is derived from Latin word ‘viron’ which means ‘poison’. Virus is the smallest entity, which is parasitic, ultramicroscopic and disease producing organism. Virus multiplies only inside the living cells of plants and animals and for this reason, is called obligatory parasite. In non-living virus is inactive but it becomes active if it infects a living cell. It multiplies in a living cell and cause many diseases in living organism. Thus, virus is

extremely small, infectious pathogens that can multiply within living cell only. 

Virus was first isolated by Iwanowski in 1892 who showed that the mosaic disease in tobacco could be caused by the virus called Tobacco Mosaic Virus (TMV). Virus is so small that it passes through the fine pores of a clay filter. The size of viruses ranges from about 25 nanometer to 250 nanometers. They are found 50 times smaller than bacteria. They have different shapes. They can be found in different shapes like rod, spherical, hexagonal, polyhedron, etc. they infect both plants and animals. A large number of viruses infect human beings and cause diseases such as small pox, common cold, measles, mumps, rabies, poliomyelitis, avian flu, swine flu and AIDS. Nowadays, covid-19 is rapidly spreading all over the world due to which thousands of people have loosed their life. The study of virus is called virology.

Characteristics of Viruses

• Viruses do not show any cellular structure.
• They are like molecule.
• They multiply only inside the living cells.
• They are dead outside the cells and can be crystallized.
• They contain RNA (Ribonucleic acid) or DNA (Deoxyribonucleic acid). These nucleic acids allow them to transfer hereditary characteristics to their offspring.
• They replicate themselves.
• They live as parasites in plants and animals.
• They attack different cells, tissues and cause many diseases.

Viruses show characteristics of both living as well as non-living things. Therefore, they can be considered as border line between living and non-living things.

Structure of Viruses

Viruses have different shapes. They are extremely small organisms. They have a simple non cellular structure. They are referred to as particles. They have no cytoplasm, nucleus and cell membrane. The virus consists of nucleic acid, which is either RNA or DNA surrounded by a protein coat. The protein coat protects the nucleic acid.

Classification of Viruses

On the basis of host, viruses are of different types:

Animal viruses: The viruses which attack animals or live as parasites on animals are called animal viruses. There are various types of viruses which cause diseases such as polio, mumps, chicken pox, measles, encephalitis, dengue fever, AIDS, etc. Rhinovirus, retrovirus, polio and rhabdo viruses are examples of animal virus.

Plant viruses: The viruses that attack plants or live as parasites on plants are called plant viruses. There are three hundred types of plant viruses which cause diseases in crops such as apple, plum, almond, potato, tomato, tobacco and cocoa. Tobacco mosaic virus, cucumber mosaic virus and bean yellow mosaic virus are examples of plant viruses.

Bacteriophages:

The viruses which infect bacteria are called bacteriophage. It has a tadpole like structure. It has a hexagonal head and cylindrical and hollow tail. The head is called capsid containing protein called capsomere. It encloses DNA molecule. The tail contains six tentacles called tail fibers.

On the basis of nucleic acid, viruses are of two types:

RNA viruses: Rhinovirus, retrovirus, polio virus, mosaic virus, etc.

DNA viruses: Bacteriophages, small pox virus.

Mode of Transmission of Viral Diseases

The particular virus attacks particular organism and particular organs. So there are different ways to transmit viral diseases in plants and animals.

In plants: There are different ways to transmit viral diseases from one plant to another plant.

• By rubbing the virus on the leaves.
• By grafting between a diseased and a healthy plant
• By insects like aphids. It is transmitted during feeding the plant sap.
• Certain virus like wheat mosaic virus is transmitted from the soil.

In animals: There are different ways to transmit viral diseases in animals.

• In animals and human beings, viruses are transmitted through direct contact or through spitting and sneezing and coughing.
• In animals and human beings, viruses are transmitted through faeces, contaminated food and water.
• By close contact between infected and healthy persons through cuts and wounds.

Control of Viral Diseases

There are several methods to control the viral diseases in plants and animals.

• The best method to control the viral diseases of a plant is to burn the diseased plant.
• By destroying the insects like aphids.
• Human beings can be protected from viral diseases by the use of vaccines.