Virus Definition

What are Viruses?

In biology, viruses are microscopic entities that are able to make their replica inside the cell of its host. The definition of viruses differs among scientists because they cannot be classified as living organisms or non-living objects. This is because the viruses have certain characteristics of living organisms as well as non-living entities.

Viruses can replicate like living organisms but they can only do so when they are inside the host’s cell. However, viruses do not contain the necessary cellular components that are important for life.

Hence, the cellular machinery of the virus's host has to be "hijacked" to produce copies of itself. Unlike living organisms, viruses may be crystallized and may remain active.

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Origin of Virus

Wherever there is life and have probably existed since living cells first evolved, we may find viruses are there. The origin of viruses is not so clear because they do not form fossils, therefore molecular techniques are used to investigate how they came into origin. Moreover, viral genetic material occasionally integrates into the germline of the host organisms, by which they can be passed on vertically to the offspring of the host for many generations. An invaluable source of information is provided for paleo virologists to trace back the ancient viruses that have existed on this earth millions of years ago.

Role of Viruses in Human Disease

Few examples of common human diseases that are caused by viruses are the common cold, chickenpox, influenza, and cold sores. There are also some serious diseases caused by viruses such as rabies, AIDS(HIV), Ebola virus disease, SARS, and avian influenza are caused by viruses. The relative ability of viruses to cause disease is popularly known as virulence. Other diseases are under analysis to find out whether they have a virus as the causative agent, such as the valid link between human herpesvirus 6 (HHV6) and neurological disorders like multiple sclerosis and chronic fatigue syndrome. There is contention over whether the bornavirus, which was once thought to cause neurological diseases in horses, could now be responsible for psychiatric illnesses.


Bacterias are very small single-celled microbes. The cell structure of a bacteria is simpler than that of any other organism as there is no nucleus or membrane-bound organelles. The control center of bacterias containing the genetic information is contained in a single loop of DNA. Some bacteria have an extra circle of genetic material that are double-stranded DNA molecules called a plasmid. The plasmid often contains genes that give the bacterium some superiority over other bacteria. It might contain a gene that makes the bacterium resistant to some antibiotics.


Genes are the working subunits of DNA. Every gene comprises a particular set of instructions, usually coding for a specific protein or for a specific function. The DNA in each chromosome constitutes many genes. In order to make proteins, the gene from the DNA is copied by each of the chemical bases into messenger RNA (ribonucleic acid) or mRNA.

Variant form of a given gene is known as an allele. Some genes may have different forms, that are located exactly at the same position often called a genetic locus, on a chromosome. Humans are known as diploid organisms because they have two alleles at each of the genetic loci, with an allele inbred from each of the parents. Each pair of alleles represents the genotype of a particular gene. Genotypes are defined as homozygous if there are two identical alleles at a certain locus and as heterozygous if the two alleles differ. Alleles also contribute to the phenotype of an organism, which is the outward appearance of the organism.

Did You Know?

  • DNA or RNA; single or double-stranded; maybe the genetic information of the viruses

  • The term 'virus' was coined from the Latin word that means slimy liquid or poison.

  • Walter Reed was the first to discover the first human virus, yellow fever virus, in 1901.

  • Viruses are not living– they are lifeless complex organic substances. They need any form of energy, carbon metabolism moreover they cannot replicate or evolve.

  • Within the cells, viruses reproduce and evolve.

  • Over 1016 human immunodeficiency virus genomes are created daily on the planet. As a result, thousands of viral mutant quotidian arise by the probability that is resistant to every combination of antiviral compounds in use or in improvement.

  • Mimiviruses, which are 400 nanometers (0.0004 millimeters) in diameter, is the biggest virus known yet.

  • The viral genome of mimivirus is about 1,200,000 nucleotides in length and codes for over 900 proteins.

  • Circoviruses, which are 20 nanometers (0.00002 millimeters) in diameter, is the smallest virus yet known.

FAQ (Frequently Asked Questions)

1. Do Viruses Possess Features like Cellular Respiration, Photosynthesis, and Reproduction?

Ans: Viruses do not have cellular respiration, because they are not cells, which is why antibiotics do not affect viruses. 

Viruses do not photosynthesize, because they do not have chloroplasts or chlorophyll, whatsoever. 

Viruses do reproduce but in a different manner. They immunize their genetic substance into a cell, commandeering its machinery to mass-produce replicas of the actual virus before lysing (dissolving the cell membrane) and discharging these viruses to scatter and infect more cells, which give rise to further viruses which also infect more cells. 

Thus we may say that Viruses can reproduce but they do not possess any features such as cellular respiration and photosynthesis.

2. What are Genetic Engineering and Genetic Counseling?

Ans: Genetic engineering is a technique in which the genetic composition of an organism is changed by introducing new genes into its chromosomes. The genetically modified organism which is produced is then grown to multiply fast and the gene product is obtained in huge quantities. Insulin is the hormone that was the first such product.

Newly married couples are counseled by a specialist regarding the possibilities of any undesirable trait that their baby might inherit.

Diseases like hemophilia, thalassemia, and sickle cell anemia with defective hemoglobin are examples of genetic diseases that can be prevented to some extent by proper genetic matching of the prospective parents.