What is a Virus?

In biology, a virus is neither a living nor a non-living entity. 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.

Virus Diagram

Diagram of a Virus: (Left) A virus that infects bacteria is called a bacteriophage. (Right) HIV has a long incubation period – roughly 10 years.

These microbes belong to the family viridae and genus virus. 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. The nucleic acid can have single or double strands. The whole infectious virus, called as virion has nucleic acid and an outer shell of protein. The simplest virus includes DNA or RNA for encoding four proteins and the most complex encodes 100-200 proteins.

The study of viruses is called as virology.

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 with a protein coat – capsid and have a nucleic acid core comprising of 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.

Classification of Viruses

The term “virus” is also used in Information Technology to describe malicious programs that show some characteristics of biological viruses. For instance, they spread from computer to computer, similar to how biological viruses spread from host to host.  Viruses can be classified primarily on their phenotypic characteristics such as:


Viruses are classified based on the chemical composition, size and shape, genome structure and modes of replication. Helical morphology is found in nucleocapsids of different pleomorphic and filamentous viruses. Helical nucleocapsids include a set of helical capsid proteins coated over a filament of nucleic acid. Many capsomeres arrangement are effective in the identification and many consists of an outer envelope as well.

Chemical Composition

The whole genome may cover either one molecule of nucleic acid (monopartite genome) or different segments of nucleic acid (multipartite genome). The different kinds of genome call for various replication techniques.

Structure and Function

Viruses are tiny, obligate intracellular entities that either has DNA or an RNA genome. It is also 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. 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 is packed in a capsulated symmetric protein. The protein associated with nucleic acid (Also known as nucleoprotein) produces the nucleocapsid with the genome.

Types of Viruses In Biology

Based on the type of host, three types of viruses have been identified:

  1. Animal viruses
  2. Plant viruses
  3. Bacteriophage

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 etc.

Plant viruses

These viruses infect plants by invading the plant cells. Well-known examples of plant virus include potato virus, tobacco mosaic virus, beet yellow virus and turnip yellow virus etc.


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

List of Viral Diseases

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

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

Overview of Viruses

Viruses are microscopic infectious agents that can only replicate inside a host cell. From a biological perspective, viruses cannot be classified either as living organism nor non-living. This is due to the fact that they possess certain defining characteristic features of living organisms and non-living entities. For instance, a virus cannot replicate itself outside the host cell. This is due to the fact that viruses lack the required cellular machinery and hence, it has to highjack other living cells to replicate itself. 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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Important Questions about Viruses

1. 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.

2. 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.

3. 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.

4. State a few examples of viral diseases.

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

5. 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 crystalized. During this stage, they remain dormant, until they enter another host, restarting the cycle all over.

Practise This Question

The virus envelope is also known as _____.