“Viruses are infectious agents that replicate inside the body of a host.”

Table of Contents

What is a Virus?

Viruses are non-cellular, microscopic infectious agents that can only replicate inside a host cell. From a biological perspective, viruses cannot be classified either a living organism or non-living. A virus can be an infectious agent which only replicates within a host organism. This is due to the fact that they possess certain defining characteristic features of living organisms and non-living entities.

In a nutshell, a virus is a non-cellular, infectious entity made up of genetic material and protein that can invade and reproduce only within the living cells of bacteria, plants and animals.

For instance, a virus cannot replicate itself outside the host cell. This is because viruses lack the required cellular machinery. Therefore, it enters and attaches itself to a specific host cell, injects its genetic material, reproduces by using the host genetic material and finally the host cell splits open, releasing the new viruses.

Viruses can also be crystallized, which no other living organisms can do. It is these factors that lead to viruses being classified in the grey area – between the living and non-living.

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Also Read: Cytopathic Effect

Structure and Function of Viruses

Viruses are tiny and smaller in its size, ranging between 30-50 nm. Viruses do not contain cells and usually lack a cell wall but are surrounded by a protective protein coating called the capsid. It can be seen as a genetic element and is characterized by the combined evolution of the virus and the host. They contain either RNA or DNA as the genetic material.

Viruses mainly depend on a host to deliver the complex metabolic machinery of prokaryotic or eukaryotic cells for propagation. The main task of the virus is to carry its DNA or RNA genome to the host cell, which then can be transcribed by the host cell. The viral genome structure is packed in a capsulated symmetric protein. The protein associated with nucleic acid (also known as nucleoprotein) produces the nucleocapsid with the genome.

Explore: Difference Between Virus And Bacteria

Virus Diagram

Bacteriophage and HIV

These microbes belong to the family viridae and genus virus.  Viruses could not be placed in any of the kingdoms because they are practically neither living nor dead. The term virus was coined by the Dutch microbiologist, Martinus Willem Beijerinck in the year 1897. It is derived from Latin, which means poison or venomous substance.

Once a susceptible cell is infected, a virus can start the cell machinery to generate more virus. Viruses are composed of a core of DNA or RNA surrounded by a protein coat. They are very small and their size ranges from 20 nanometers to 250 nanometers. Therefore, they can only be seen with an electron microscope

Many viruses have either DNA or RNA as the genetic element and the nucleic acid with single or double strands. The whole infectious virus, called as virion has nucleic acid and an outer shell of proteins. The simplest virus includes DNA or RNA for encoding four proteins and the most complex encodes 100-200 proteins.

The study of virus is known as virology.

Further Reading: Microbes

Properties of Viruses

  1. They are non-cellular organisms, which is enclosed in a protective envelope.
  2. The presence of spikes helps in attaching the viruses to the host cell.
  3. These viruses do not grow, neither respire nor metabolize, but they reproduce.
  4. They are surrounded by a protein coat – capsid and have a nucleic acid core comprising DNA or RNA.
  5. They are considered both as living and non-living things. These viruses are inactive when they are present outside of host cells, but become active within host cells. These viruses cause several infections and reproduce within the host cell by using the enzymes and raw materials.

Also Read: Creutzfeldt-Jakob Disease

Classification of Viruses

Viruses can be classified primarily on their phenotypic characteristics, core content, chemical composition, capsid structure, size, shape, modes of replication and other viral genome structures.

The Baltimore classification is the most commonly used for studying the system of virus classification. This system was developed by an American biologist David Baltimore in the 1970s, for which he was awarded the Nobel Prize.

The below virus information describes the classification of viruses based on their different criteria.

Classification based on the presence of nucleic acid

DNA virus

The virus, having DNA as its genetic material. There are two different types of DNA virus

Single-stranded (ss) DNA virus: e.g. Picornaviruses, Parvovirus,  etc.

Double-stranded (ds) DNA virus: e.g. Adenovirus, Herpes virus, etc.

RNA virus

The virus, having RNA as its genetic material. There are two different types of RNA virus

Double-stranded (ds) RNA virus: e.g. Reovirus, etc.

Single-stranded (ss) RNA virus. It is further classified into two Positive sense RNA (+RNA) and negative sense RNA (-RNA). Poliovirus, Hepatitis A, Rabies virus, Influenza virus are examples of single-stranded RNA virus.

Classification based on the structure or symmetry

Viruses come in different shapes, from basic helical and icosahedral shapes to more intricate ones. The classification based on different shapes and symmetry of viruses are as follows:

  1. Complex virus. E.g Poxvirus
  2. Radial symmetry virus. E.g.Bacteriophage
  3. Cubical or icosahedral symmetry shaped virus. E.g. Reovirus, Picornavirus
  4. Rod or Spiral shaped or helical symmetry virus. E.g. Paramyxovirus, orthomyxovirus

Read more: Virus Nomenclature

Classification based on the replication properties and site of replication

Here, viruses invade into the host cell, where it replicates and assembly within the cell organelles.

  1. Replication within the cytoplasm of the host cell.
    E.g. All RNA viruses except the Influenza virus.
  2. Replication within the nucleus and the cytoplasm of the host cell.
    E.g. Influenza virus, Poxvirus, etc.
  3. Replication within the nucleus of the host cell.
    All DNA viruses except Pox virus.
  4. Replication of the virus through the double-stranded DNA intermediate.
    E.g. All DNA viruses, Retrovirus and some tumour causing RNA virus.
  5. Replication of the virus through a single-stranded RNA intermediate.
    E.g. All RNA viruses except Reovirus and tumour-causing RNA viruses.

Read more: Escherichia virus T4: Structure, Life Cycle and FAQs

Classification based on the host range

Based on the type of host, there are four different types of viruses:

Animal viruses

These viruses infect by invading the cells of animals, including humans. Prominent examples of animal viruses include the influenza virus, mumps virus, rabies virus, poliovirus,  Herpes virus, etc.

Plant viruses

These viruses infect plants by invading the plant cells. Replication of plant viruses is obligate and does not happen without a host. Well-known examples of plant virus include the potato virus, tobacco mosaic virus (TMV), beet yellow virus, and turnip yellow virus, cauliflower mosaic virus, etc.


The virus which infects bacterial cells is known as bacteriophage. There are many varieties of bacteriophages, such as DNA virus,  MV-11, RNA virus, λ page, etc.

Explore more: Bacteriophage

Insect virus

The virus which infects insects is known as Insect virus, also called the viral pathogen of insects. These viruses are considered as a powerful biocontrol agent in the landscape of modern agriculture. Ascovirus virions and  Entomopox virus, are best examples for insect virus.

Classification based on the mode of transmission

  1. Airborne infections – Transmission of the virus through the air into the respiratory tract. E.g. Swine flu, and Rhinovirus.
  2. Fecal oral route  – Transmission of the virus through the contaminated water or food.
    E.g. Hepatitis A virus, Poliovirus, Rotavirus.
  3. Sexually transmitted diseases – Transmission of the virus through sexual contacts with the infected person. E.g. Retrovirus, human papillomavirus, etc.
  4. Transfusion-transmitted infections- Transmission of the virus through the blood transfusion.
    E.g. Hepatitis B virus, Human Immunodeficiency Virus, etc.
  5. Zoonoses -Transmission of the virus through the biting of infected animals, birds, and insects to human. E.g. Rabies virus, Alpha virus, Flavivirus, Ebola virus, etc.

Also Read: Virology

Virus Reproduction

Lytic infection is the method used by the majority of viruses to reproduce. A virus penetrates the host cell during lytic infection, replicates, and causes the cell to lyse or explode.

Overview of the lytic cycle:

  • Attachment: Virus binds to host cell during attachment.
  • Entry: Injection of genetic material into the host cell.
  • Replication: The virus takes control of the host cell’s metabolism, leading the organelles to produce new proteins and nucleic acids.
  • Assembly: Nucleic acids and proteins are put together to form new viruses during assembly.
  • Release: Viral enzymes induce the host cell to burst, releasing viruses into the surrounding environment. These novel viruses are capable of infecting other cells.

List of Viral Diseases

Following is a list of virus diseases that have made a significant socioeconomic impact in the last few decades.

  • AIDS (Acquired Immunodeficiency Syndrome)
  • Ebola
  • Influenza
  • SARS (Severe Acute Respiratory Syndrome)
  • Chikungunya
  • Small Pox (Now eradicated)

Explore more about Viral Diseases

Explore more about viruses or other related concepts by register at BYJU’S Biology.

Economic Importance of Virus

Viruses are used in biotechnology research because they share the properties of living and non-living species. The viruses can be both helpful and harmful. Bacteriophage can be used to preserve water since it can eliminate germs and maintain the freshness of the liquid.

  • Pox, polio, mumps, jaundice, and other diseases can be controlled by injecting dead viruses into people as vaccines, which is how antidotes and vaccines are made.
  • A specific virus can control some insects and animals that are hazardous to people.
  • Disease management: The T2 bacteriophage virus protects from dysentery by killing dangerous bacteria, such as E-coli. Because viruses can specifically target cells and DNA, they are used in virotherapy to treat various disorders. It might play an essential role in gene therapy and cancer treatment.
  • The most familiar living model utilised in laboratories is the virus. In genetics research, viruses are primarily used. It is an essential topic of discussion in genetic engineering.
  • Due to the virus’s combination of living and non-living traits, it is necessary to understand the evolutionary tendency and the mechanism by which living entities are created.
  • Viruses are an example of an organic nanoparticle in nanotechnology. They have been utilised as a model for arranging materials on the nanoscale due to their shape, size, and structures.
  • One million viruses can be found in a spoonful of seawater, aquatic ecosystems’ most abundant natural component. A virus can boost the number of photosynthesis in oceans and reduce the quantity of carbon dioxide in the atmosphere by about three gigatonnes of carbon per year.

Frequently Asked Questions About Viruses


What is a Virus in Biology?

A virus is a biological entity that can only reproduce within a host. Anatomically, viruses possess nucleic acids (DNA or RNA) which are encased within a  protective protein coat. These entities are able to infect all forms of life, ranging from bacteria to humans, and consequently, they bring about a multitude of diseases in their host.


State the basis of classification of viruses.

Typically, classification of viruses mainly depends on their phenotypic characteristics such as morphology, chemical composition, structure, and function.


Name two DNA viruses.

Two DNA viruses include herpesviruses and papillomaviruses, among many others.


Give 5 facts about viruses.

The following list of five notable facts about viruses includes:

  • Viruses can infect bacterial, plant, and animal cells
  • Retroviruses are used in Gene Therapy and Cloning
  • No other living being evolves as quickly as viruses do
  • Many viruses can lead to cancer
  • For years, a virus may stay dormant in a host

List the types of viruses In biology.

Based on their host, viruses can be classified into three types, namely, animal viruses, plant viruses, and bacteriophages.


State a few examples of viral diseases.

  • AIDS
  • Chikungunya
  • Ebola
  • Influenza
  • SARS
  • Small Pox

Why are viruses neither considered living, nor non-living?

Viruses possess trademark characteristics of both living and non-living entities. For instance, they can only reproduce within a host, just like a parasite. But unlike parasites or any other living organisms, viruses can be crystallized. During this stage, they remain dormant, until they enter another host, restarting the cycle all over.

Also check: History of Virus

Test your Knowledge on Virus


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