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.
Bacteria
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
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.
FAQs on Virus Definition
1. What is the simplest definition of a virus?
A virus is a sub-microscopic, infectious agent that can only replicate inside the living cells of other organisms. Viruses are obligate intracellular parasites, meaning they lack the cellular machinery for self-reproduction and must hijack a host cell's resources to multiply. They are composed of genetic material, either DNA or RNA, enclosed in a protective protein coat called a capsid.
2. Why are viruses considered to be on the boundary between living and non-living things?
Viruses are considered to be on the boundary of living and non-living because they exhibit characteristics of both. When outside a host cell, a virus is an inert, non-living particle called a virion. However, once it infects a living cell, it uses the host's machinery to reproduce, a key characteristic of life. They lack their own metabolism and are not made of cells, which are traits of non-living entities.
3. How is a virus fundamentally different from a bacterium?
The fundamental differences between a virus and a bacterium are:
- Structure: Bacteria are complete, single-celled organisms with cellular machinery like ribosomes. Viruses are much simpler, consisting only of genetic material and a protein coat, and are not cells.
- Size: Viruses are significantly smaller than bacteria and can only be seen with an electron microscope.
- Reproduction: Bacteria reproduce independently through binary fission. Viruses are obligate parasites that must invade a host cell to replicate.
- Treatment: Bacterial infections are treated with antibiotics, which are ineffective against viruses. Viral infections are managed with antiviral medications and vaccines.
4. What is the basic structure of a virus?
The basic structure of a typical virus consists of two main components: a core of genetic material (either DNA or RNA, but never both) and a protective outer protein shell called a capsid. The capsid is made of smaller protein subunits called capsomeres. Some viruses, known as enveloped viruses, have an additional outer lipid membrane derived from the host cell.
5. How are viruses classified?
Viruses are primarily classified based on several criteria as per the guidelines for the 2025-26 syllabus. The key factors include:
- Type of Genetic Material: Whether they contain DNA or RNA, and if it's single-stranded or double-stranded.
- Structure of the Capsid: The shape and symmetry of the protein coat (e.g., helical, icosahedral).
- Presence of an Envelope: Whether they are enveloped or non-enveloped (naked).
- Host Range: The type of organisms they infect (e.g., bacteriophages infect bacteria, animal viruses, plant viruses).
6. Can a virus reproduce on its own? Explain the process.
No, a virus cannot reproduce on its own. It is an obligate intracellular parasite and must infect a host cell to replicate. The process involves several steps: the virus attaches to a host cell, injects its genetic material, takes over the host's metabolic machinery to synthesise new viral proteins and genetic material, assembles these components into new virions, and finally, causes the cell to lyse (burst) to release the newly formed viruses.
7. Who is credited with the discovery of viruses?
The discovery of viruses began with Dmitri Ivanovsky, a Russian botanist, who in 1892 showed that the agent causing tobacco mosaic disease was smaller than a bacterium because it could pass through filters that retained bacteria. Later, in 1898, Martinus Beijerinck named this new infectious agent 'virus', which is Latin for 'poisonous fluid'.
8. What are viroids and how do they differ from viruses?
A viroid is an infectious agent that is even simpler than a virus. The key difference is that a viroid consists only of a short, single-stranded, circular RNA molecule and completely lacks a protein coat (capsid). Viruses, in contrast, always have a capsid protecting their genetic material. Viroids are primarily known to cause diseases in plants.
9. What are some examples of common diseases caused by viruses in humans?
Many common human diseases are caused by viruses. Some prominent examples include:
- The common cold (caused by Rhinoviruses)
- Influenza (caused by Influenza viruses)
- COVID-19 (caused by SARS-CoV-2)
- AIDS (Acquired Immunodeficiency Syndrome, caused by HIV)
- Measles and Mumps
- Poliomyelitis (Polio)
