Definition Of A Virus | Structure, Viral Taxonomy, Types of Viruses, Symptoms, Transmission, Control (2024 Guide)

The definition of a virus was composed at different times by different workers. Its understanding can be depicted by the definition list. As we know, viruses emerged as a new identity around the 1930s.

Definition Of A Virus By Different Scientists

Green (1935)

According to Green’s definition of a virus, ‘they are the smallest units showing the reproductive property considered typical life’.

Stanley (1938)

Stanley says, “We are forced to conclude, therefore, that although tobacco mosaic virus (T.M.V) protein has the ordinary properties of molecules, it also has the ability to reproduce and to, mutate properties not ordinary ascribed to molecules and hence that T.M.V protein represents are entity unfamiliar to us”.

Bawden (1943)

Bawden said that ” virus is a pathogen which consists of at least one dimensions of less than 200 millimicrons – mu and in nature, it is obligatory and parasitic)”. This is the latest and most accepted definition of a virus.

Today’s Virion Definition

Now a day the acceptable definition of a virus says, they are obligate intracellular parasites and vary from 20-20 nm in size. They have varied shapes and chemical compositions but contain only RNA and DNA.

The intact particle is termed a “virion” which consists of a capsid that may be enveloped further by a glycoprotein/lipid membrane and are resistant to antibiotics.

Structure of a Virus

What Is The Basic Structure Of A Virus?

The basic structure of a virus consists of:

  • Physical Nature
  • Chemical Nature

Physical Nature Of Virus

According to the definition of a virus, they are recognized as extremely small that cannot be seen through the highest magnification of the microscope using visible light.

They are only recognizable by their biological behavior, such as the disease they cause. They were regarded as an invisible form of bacteria, enzymes, toxins, or as unusual products of metabolism of the cells in which they were found.

Chemical Nature Of Virus

An American biochemist, Stanley in 1935. isolated a material by chemical means from the diseased tobacco leaves, which appeared to be a protein of high molecular weight.

By studying the found material, Stanley observed that it incorporates all properties of the tobacco mosaic virus. Stanley on the basis of the chemical properties identified the material as autocatalytic protein, which can multiply in a living cell.

Bawden purified the tobacco mosaic virus and found it to be a crystalline nucleo-protein of very high molecular weight, retaining its infectiousness even when diluted to a concentration of 1/1,000,000.

This virus when examined with the recently developed electron microscope using X-rays was found to be in the form of bundles of rod-like protein. It must be a virus itself because such nucleoproteins cannot be obtained from healthy plants.

The study of the potato virus confirmed this in 1938. Since then, protein of high molecular weight possessing all the properties of the respective virus has been isolated and studied.

Viruses have a core of DNA and a coat of protein in case the host is animal or bacterial while in the case of plant viruses the coat is made of protein but the core is made of RNA.

Virus Structure Diagram

Diagrammatic representation of four types of virus partials

Rabies Virus Structure Diagram
HIV Virus Structure Diagram
Adeno Virus Structure Diagram
Herpes Simplex Virus Structure Diagram

Size Range Of Viruses

Viruses are differentiated in size but many of them are rod-like and crystalline. The tobacco mosaic virus has a rod-like structure. Its length is 280 and its breadth is 18.

The tomato virus known as the ‘Bushy stunt’ virus has a diameter of 274 and 8,800,000-12,800,000 in molecular weight. Stanley studied the nucleo-proteins of the ‘ring spot’ virus of tobacco.

The virus with 19 diameter and molecular weight of 3,400,000. The smallest plant virus known so far is Tobacco ‘necrosis ‘ virus. It is 13-20 in diameter and rounded in shape.

Viral Taxonomy

Taxonomic Classification Of Viruses

The classification of viruses involves the use of a wide range of characteristics, which includes three basic properties.

Virus Properties

a) Morphology which is further categorized into, size shape presence or absence and nature of peplomers, presence, and absence of envelope, capsid symmetry, and structure.

b) Physiochemical and physical properties which are further categorized into molecular mass, sedimentation coefficient, PH stability, thermal stability, cation stability, solvent stability, detergent stability, irradiation stability, etc.

c) Genome is further categorized into type of nucleic acid, size of the genome, linear or circular, number and size of segments, nucleotide sequence, presence of isomerization, etc.

d) Proteins are further categorized into a number, size, and functional activities of structural and non-structural proteins. Amino acid sequence, glycoprotein, etc.

e) Lipids are further categorized into contents and characters etc.

f) Carbohydrates are further categorized into contents and characters etc.

g) Genome organization and replication are further categorized into genome organization, the strategy of replication, number and position of open reading frames, transcription characters, site of accumulation of virus proteins, site of virus assembly, etc.

Anti-Genetic Properties

Serologic relationships, especially as obtained in reference centers.

Biological Properties

Natural hot range, mode of transmission in nature, vector relationship, distribution, pathogen city, association with disease, tissue tropism, pathology, histopathology, etc.

Viruses In The Taxonomic Hierarchy

Based on these parameters’ the hierarchy developed for the virus is given below.

Virus Orders

These represent groupings of families of viruses that share common characteristics which make them distinct from other orders and families.

Orders are distinguished by the suffix –verales. One order has been approved by the ICTV: Mononegavirales, which consists of the families Paramyxoviridae, Rhabdoviridae, and Filoviridae.

Virus Families

These represent groupings of genera of viruses that share common characteristics and are distinct from the members of other families. Families are designated by the suffix —viridae.

Virus Genera

Virus genera are groupings of species of viruses that share common characteristics and are distinct from the members of other genera. They are designated by the suffix virus. e.g., Genera Simplex virus and Varicella virus.

The criteria for designating genera vary from family to family but include genetic structural and other differences.

Virus Species

A virus species is defined as “a polythetic class of viruses that constitutes a replicating linage and occupies a particular ecological niche”. Members of a polythetic class are defined by more than one property.

At present, the ICTV is examining the properties which can be included in determining species. The division between species and strains is a difficult one.

What Are The Three Types Of Viruses?

At present most virologists agreed that viruses are nucleo-proteins of high molecular weight. The coding capacity of viral genomes varies from <5 to >100 genes.

The Grouping Of Viruses Is Based Partly On The Following Reasons:

Viral proteins are either structural, present as part of virion architect, or non-structural, present only in infected cells. The limited number of virus proteins synthesized means that many of them are multi-functional.

They have the power of multiplication when in some appropriate host. They are so restricted to the host that they are grouped according to their host.

That’s why they are divided into three main groups, such as:

  • Bacterial Viruses
  • Animal Viruses
  • Plant Viruses

Bacterial Viruses

These are viruses that specifically attack bacteria. These bacterial viruses are also called bacteriophages. Many scientists refer to these viruses as T4 bacteriophages.

Now, the t4 bacteriophage has a complex shape and mechanism by which it attacks the bacterial cell. Studies show that these viruses are like a tadpole in morphology.

Viruses Definition Bacteriophage T4

Animal Viruses

These are the types of viruses that attack animals and cause diseases in them. Some important animal viruses are:

  • Influenza virus
  • Mumps
  • Poxvirus
  • Reovirus, etc.

In structure Influenza virus differs from Tobacco Mosaic Virus (TMV) in that instead of being straight rigid particles, they are apparently more flexible and form loose coils.

A further characteristic is that the DNA supercoils are enclosed in a protein envelop the outside surface of which is covered with minute protein particles probably involved in attaching the virus to the surface of host cells.

Viruses Definition Influenza Virus

Plant Viruses

There are over 1000 plant viruses that have been classified by the ICTV. The virus usually receives its name by a combination of the host and the type of disease produced.

Plant viruses are diverse in their morphology, nucleic acid composition, and replicating patterns. The list of important plant viruses includes:

Generally, all crystallized plant viruses are composed of ribonucleoproteins. Below is an example of plant viruses.

Tobacco Mosaic Virus Diagram

Plant Viruses Symptoms, Transmission, And Control

Here we’ll be discussing plant viruses in detail. What are their symptoms, How do they transfer, and what are ways to eradicate them:

Symptoms of Viruses, Transmission, and Control of Viruses

Symptoms Of Plant Diseases Caused By Viruses

Symptoms of viruses are of a vast variety. Sometimes the same virus can cause widely different symptoms on different host plants, and the symptoms may sometimes be produced by a mixture of two or more viruses on the same plant.

Thus, the viruses can be recognized easily by the symptoms they produce on the host. A few more common and easily detectable symptoms are given here:

Chlorosis In Plants

Due to the presence of viruses the chlorophyll of the green organs disappears at places, leaving yellowish spots, this is known as chlorosis, the presence of yellow spots at places, in the green tissue appears like a ‘mosaic’ pattern, and therefore, the diseases with such symptoms. are known as ‘mosaic’ diseases.


When the chlorophyll disappears completely from the host tissue, the organs turn yellow and the symptom is known as ‘yellows‘. Mostly found in the leaves of plants, when they turn yellow. These are called yellow plant leaves.

Vein Clearing And Vein Banding

The disappearance of chlorophyll along the veins of the leaves is known as vein clearing and when chlorophyll surrounding the veins disappears the symptom is known as vein banding.

Necrosis In Plants

The brownish sports due to the death and ultimate drying of the tissue are known as necrotic spots and the phenomenon is known as necrosis.

Necrotic spots are also known as lesions. Ring spots, bunchy top, galls hypertrophy, atrophy, rolling, curling, crinkling of leaves, stunting, and dwarfing of plants are various other symptoms usually produced on the hosts by viruses.

Transmission Of Plant Viruses

It is defined as the movement/transfer of the virus from infected plants to healthy ones by a number of agencies. These agents can be found in the surrounding area. The list is mentioned below.

By Grafting

A large number of plants are vegetatively propagated. The most common method used in propagating fruit and ornamental plants is Grafting.

When a virus-infected plant is utilized for grafting with a healthy plant, the virus is transmitted through the cell solutions flowing from the infected part into the healthy parts.

By Seeds

Seed transmission of viruses is not common. Cucurbits and legumes viruses are transmitted by seeds. Their viruses are known as Mosaic Viruses.

But in the crops, the viruses are transmitted by organs. Because crops are vegetatively propagated by the use of setts, tuber, rhizomes, bulbs, corms, etc.

By Contact

Due to contact or slight rubbing of the infected and healthy plant organs, the viruses can be transmitted. Such transmission is quite easy in a thickly populated field where one plant is always in close contact.

The organs can be rubbed with one another with the help of the slightest movement of the wind. Viruses usually gain entry through the injuries caused on plant surfaces.

By Air And Water

The viruses are not easily transmitted by air and water, unlike fungi and bacteria. The tobacco necrosis viruses have been observed to be transmitted by both air and water.

By Soil

The timing of TMV (Tobacco Mosaic Virus) is amazing. It lies dormant in the soil with the debris of plants after the crop harvest. When new seeds are sown in the same field where TMV is present. The virus infects the newly grown crop. Thus, the cycle continues, if precautionary measures are not taken.

By Tolls And Agricultural Operations

The tools or hands of the field workers also get affected/contaminated by the juice of infective plants during agriculture operations when the infected plant is cut or damaged i.e., pruning, weeding, irrigation, or topping.

Now, these tools and the hands of the worker transfer the viruses to the healthy plants, the contact with which cannot be avoided ordinarily.

By Store House

In storehouses, where the infected leaves or any infected part of the plant is stored. Healthy plants shouldn’t be stored there. Because viruses such as TMV transfer from the infective plant or leaves of the infective plant to the healthy ones.

By Insects

In nature, viruses are ordinarily transmitted by aphids, jessed, and whiteflies, which are sucking insects.

Most crop disease viruses are transmitted by insects, and for this simple reason, some virologists believe that there would have been no virus diseases if there were no insects. Several species of insects etc. can transfer the same virus.

Control Of Plant Viruses

The easy methods of preventing disease spread and losses due to them are as follows:


The destruction of infected plants and susceptible weeds lessens the possibility of the spread of disease.

Elimination Of Insects

The use of insecticidal dust and sprays reduces the chances of insect transmission.

Selection Of Seeds

It should be made from such fields that were free from infection. Seeds of cucurbits and legumes, Sets of sugarcane, tubers, rhizomes, bulbs, etc., should be carefully selected for seed purposes.

Tuber Indexing

It is done at the time of digging potatoes. Tubers taken from healthy plants are marked with ink and sown in small insect-proof plots. The suspected diseased plants are removed carefully leaving only the healthy plants to grow. Thus, a healthy seed is multiplied.

Resistant Varieties

They are evolved at various research centers and offer the best method for growing healthy crops.

FAQ About Viruses

Q: What Are The Three Types Of Viruses?

Ans: The viruses are divided into three main groups:
1. Bacterial Viruses
2. Animal Viruses
3. Plant Viruses

Q: What Is The Difference Between Vertical And Horizontal Transmission Of Plant Viruses?

Ans: The difference between vertical and horizontal transmission of plant viruses is that when viruses have passed from parent plants to their offspring is called Vertical Transmission. While in Horizontal Transmission the viruses pass between the same generation of plants.

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