Viruses: The connecting link between the living and non living things

Do you know what is a virus? What is the structure of the virus? In this article, you will find the definition and characteristics of the virus. You will also find the structure and economic importance of viruses.


Term virus has been originated from a Latin word Vivum which means liquid poison. Viruses are responsible for many fatal diseases in plants and animals. Causius (1867) discovered the first viral disease of plant inTulip tree and he called it as Tulip break. Swicten (1857) in Holland discovered tobacco mosaic disease. In 1891 Dimitry Ivanowsky isolated the Tobacco Mosaic Virus (TMV) by filtration technique. In 1896 M. W. Beijerinck, demonstrated that the tobacco mosaic was caused by a non-corpuscular Contagium vivum fluidum, which he called a virus. Schlesinger (1933) has been got success to isolate the virus by ultracentrifugation technique. W. M. Stanley (1935) isolated TMV in a crystalline form for which he was awarded by Nobel Prize in 1946> Bawden and Pirie (1938) have suggested the chemical structure of TMV. We can define viruses as the disease-causing agents which are smaller than bacteria and are visible under the electron microscope. Viruses are intracellular parasites with protein and nucleic acid in their structure.

Position of Viruses in Living world

Viruses have not been included in any of the five kingdom classification proposed by Whittaker. This is because most of the biologists do not consider viruses as living entities due to lack of cellular structure and metabolic activities in them. The viruses have the ability to cause fatal disease which has attracted the attention of biologists all over the world. The characteristics of both living and non-living things are found in viruses. Therefore, viruses lie on the borderline of living and non-living.

General Characteristics of Viruses

Many theories have been proposed to explain the nature of viruses. The main theories of them are; mycoplasma theory, bacterial theory, virus theory and oxidizing enzyme theory. Among them, the virus theory is the most popular and widely accepted. According to this theory, viruses are granular structures which can be seen only with the help of an electron microscope. Most of the viruses cannot be separated by a bacterial filter. All the viruses are parasites on other plants and animals. Viruses are very much specific to their host and produce specific symptoms in their host. Viruses are sensitive to humidity and temperature. Viruses are also sensitive to various mutagen and show the tendency of mutation.

Nature of Viruses

Viruses are regarded as the connecting link between the living and non-living things as they possess living as well as non-living characters.

Living characters

The living characters of Viruses are given below:
  1. Growth and multiplication takes place.
  2. Found only in the living host as parasite.
  3. Viruses can replicate inside the living cells.
  4. Viruses are made up of nucleoproteins.
  5. Viruses contain nucleic acid (DNA or RNA)
  6. Viruses can synthesize nucleic acids.
  7. Viruses possess the property of exhibiting recombination of two or more viruses together to infect the same host cell.
  8. Viruses show adaptation to various environmental conditions.
  9. Mutation takes place in viruses.
  10. Viruses cause disease in animals after infection.

Non-living characters

The non-living characteristics of Viruses are as below:
  1. Viruses can be precipitated and crystallized.
  2. These crystals may produce infection.
  3. Virus does not grow in an artificial medium.
  4. Viruses show resistance with acids and bases.
  5. No effect of sunlight.
  6. Viruses do not contain cytoplasm and enzymes.
  7. metabolism absent, thus viruses do not respire.

Structure of Viruses

Viruses are chemically nucleoprotein molecules. Viruses never have cellular structure i.e. cellular components like cell wall, plasma membrane, cytoplasm, nucleus, cell organelles and other biochemical are totally absent in Viruses. A virus or viral particle is made up of the following three parts:
  1. Envelope: An outer thin and loose cover is found around some viruses which is known as an envelope. Proteins, carbohydrates and lipids are the main constituents of the envelope. This envelope consists of many smaller protein subunits called paplomeres. Animal viruses like the influenza virus, herpes virus, Rous sarcoma virus, etc contain this type of envelope. However, some plant viruses like potato, yellow dwarf virus and bacterial virus, like pseudomonas-z also contain envelope. Viruses without envelope are known as necked viruses.
  2. Capsid: The genetic material (Nucleic acid) of the virus is surrounded by a protein coat. It covers the viral particle from all the sides. The protein coat is also known as capsid or protein envelope. Structurally, it is made up of smaller units known as capsomeres. Again, each capsomere is made up of many protein molecules. Hundreds of capsomeres may be present in the capsid of viral particle.
  3. Nucleoid : Nucleoid is the nucleic acid present inside the capsid. It is the central or core part of a viral particle. It consists of either DNA or RNA. It causes infection to suitable hosts. The genetic material in plant viruses is RNA while the genetic material in animal viruses are DNA.

Economic Importance of Viruses

Some viruses help us in different ways, these are called useful viruses while some viruses are harmful as they cause diseases. As virus forms a link between non-living and living beings, thus it helps us to understand organic evolution. Bacteriophages eat bacteria, therefore Bacteriophages are used as biocides to destroy many harmful bacteria present in a polluted area and can be used as the bioweapons. Cyanophages (Blue-green algae viruses) are used to destroy blue-green algae grown in different areas. Viruses can also be used as vectors.

The viruses cause many diseases in plants, animals and human beings. Influenza, cold, measles, polio, chickenpox, jaundice, dengue fever, AIDS, covid19, etc. are human diseases caused by viruses. Myxomatosis (rabbit), K.F.D. (monkey), rabies (dogs), foot and mouth disease (cattle), leukaemia (rat), etc. are some animal diseases which are caused by viruses. Viruses cause mosaic disease in potatoes, tobacco, bean, papaya, leaf curl in potato and papaya, bunch top in banana.


Viroids are the smallest self-replicating plant pathogens which were discovered by Diener (1971). It is the smallest single-stranded infectious RNA particles that do not contain a protein coat (Capsid). It is composed of RNA having 300 to 400 nucleotides. The main features of Viroids are:
  1. Viroids are the smallest infectious particles of RNA.
  2. Similar to RNA viruses but lack of capsid.
  3. It has low molecular weight.
  4. It can multiply by both RNA dependent and DNA dependent replication.

The viruses are transmitted from one organism to other organisms in many ways. Some of them are; by vectors, by seeds, by contact, by pollen grains, by winds, by water, by weeds, by nematodes, etc.

Frequently Asked Questions

What are the differences between bacteria and viruses?
The main differences between bacteria and viruses are as below:
  1. bacteria may be found in the free state in nature while viruses cannot be present in the free state in nature.
  2. Bacteria may be parasitic, saprophytic or symbiotic but viruses are obligate parasites and grow inside living cells only.
  3. The size of bacteria is larger and cannot pass through bacterial filter whereas viruses are ultramicroscopic and can pass through bacterial filters.
  4. Bacteria contain a cell wall made up of mucopeptides while viruses do not have a cell wall.
  5. Protein envelope is absent in bacteria but the protein envelope is present in viruses.
  6. Bacterial cell contain protoplasm but viruses do not have protoplasm.
  7. Enzymes are present in bacteria but absent in viruses.
  8. Bacteria may be motile but viruses are always non-motile.
  9. Bacteria have cell organelles but viruses do not have cell organelles.
  10. Bacteria become dead in high temperature but viruses are thermolabile.
  11. Bacteria cannot be crystallized while viruses can be crystallized.
  12. Bacteria can multiply in both living and non-living whereas viruses can multiply only in living beings.
  13. Cell division occurs in bacteria but does not occur in viruses.
What are the differences between plant virus and animal virus?
Plant virus does not have an envelope. It has only a protein covering called capsid while animal viruses are usually provided with an envelope. Plant viruses usually contain RNA as genetic material whereas animal viruses usually contain DNA as genetic materials. Some animal viruses also contain RNA.
What are the differences between viruses and viroids?
Viruses are a nucleoprotein molecule while viroid is an infectious RNA particle. The virus contains DNA or RNA but viroid contains only RNA. Capsid present around genetic material of virus whereas capsid is absent in viroid. A virus is larger in size while viroid is smaller in size. The virus infects all kinds of living beings but viroid infects only plants.
What are bacteriophages?
The viruses which attack bacteria and multiply within them are called bacteriophages. Bacteriophages are parasites on bacteria. Bacteriophages were discovered by a British scientist F. W. Twort in 1915.


Author: Venkiteswaran15 May 2020 Member Level: Diamond   Points : 7

The term Virus was mostly a matter dealt with and heard in the health and medicine related sectors. But it was more popular and familiar with the people using a computer, laptop and smartphones and generally the Software sector, though having a different meaning. The business of Anti Virus software was doing quite well and became an essential accessory for such gadgets.

Many of us would have had cursory learning included in our biology lessons in late school classes about Virus and Bacteria. But it never scared us. Then we heard it when the doctor told us that we had a viral fever. Then also we took it easy.

But the word virus really scared us out of our inertia after the Universal spread and devastation by COVID19. The media- whether print, audio-visual or the internet was full with information and misinformation about Viruses. Some of them gave real information and some were spreading misinformation.

The article here is also born out of linking the current relevance and impact. The article traces the origin of Virus discovery and goes on to some details in brief. However, the article is not very deep. Though it is not expected to be so it could certainly have been a little more detailed by tabulating the comparison or differentiation with Bacteria and other microorganisms. Though an attempt is made as FAQ, that does not serve the purpose as effectively and so should have been included in the main article itself.

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