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. 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.

In this post, Pritish Kumar Halder illustrates Virus, Structure, classification and disease caused by these viruses.

Structure and Function of Viruses

Viruses are tiny and smaller in its size, ranging between 30-50 nm. They 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 is packed in a capsulated symmetric protein. The protein associated with nucleic acid (also known as nucleoprotein) produces the nucleocapsid with the genome.

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.

Properties of Viruses

  • They are non-cellular organisms, which is enclosed in a protective envelope.
  • The presence of spikes helps in attaching the viruses to the host cell.
  • These viruses do not grow, neither respire nor metabolize, but they reproduce.
  • They are surrounded by a protein coat – capsid and have a nucleic acid core comprising DNA or RNA.
  • 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

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

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.

Different examples of virus

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

  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

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.

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

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. Well-known examples of plant virus include the potato virus, tobacco mosaic virus, 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.

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

Airborne infections – Transmission of the virus through the air into the respiratory tract. E.g. Swine flu, and Rhinovirus.

Fecal oral route  – Transmission of the virus through the contaminated water or food. E.g. Hepatitis A virus, Poliovirus, Rotavirus.

Sexually transmitted diseases – Transmission of the virus through sexual contacts with the infected person. E.g. Retrovirus, human papillomavirus, etc.

Transfusion-transmitted infections– Transmission of the virus through the blood transfusion. E.g. Hepatitis B virus, Human Immunodeficiency Virus, etc.

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.

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)