By Vidyawini Ganapathy

A virus is a submicroscopic infectious agent. Viruses have genes but no cellular structure (often considered the basic unit of life) or metabolism. They are obligate intracellular parasites, which means they can only reproduce inside the body of a host, and hence, cannot be classified as either living or non-living. They have been termed by some as “organisms at the edge of life.”

When not present in a host body, viruses exist as independent particles called ‘virions’ which consist of genetic material, a protein covering called the capsid and sometimes a lipid envelope.

Most viruses are so small that they can only be seen with an electron microscope. Many viruses are spherical and have diameters ranging from 20–200 nanometres, although some ‘giruses’(giant viruses) can exceed this range.


  • Genetic material

The group of viruses has more structural genomic diversity than plants, animals or bacteria.

Viruses can have either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) as a means to contain their biological information. Based on this, they are called DNA viruses and RNA viruses respectively. A majority of viruses are RNA viruses. RNA viruses and certain DNA viruses are segmented, i.e. the genome is divided into separate parts. For RNA viruses, each segment often codes for only one protein. Proteins associated with nucleic acid are called nucleoproteins.

Viral genomes can be single-stranded (ss) or double-stranded (ds). Single-stranded genomes have unpaired nucleic acids while double-stranded genomes have two complementary paired nucleic acids. Some viruses can also have partially single-stranded and partially double-stranded genomes, such as the viruses in the Hepadnaviridae family.

  • Capsid

A capsid (from Latin capsa, meaning ‘box’) is a protective protein coat surrounding a virion. Capsids are formed from capsomeres, protein subunits that self-assemble to form the capsid. Virus capsids display a wide variety of shapes and sizes, or 'morphologies.' Based on the shape of the capsid, viruses can be classified into five main types:

  1. Helical: They are composed of a single type of capsomere stacked around a central axis to form a helix. The genetic material is bound to the protein helix by interactions between the negatively charged nucleic acid and positive charges on the protein. Overall, the length of a helical capsid is related to the length of the nucleic acid, and the diameter is dependent on the size and arrangement of capsomeres. The tobacco mosaic virus is a helical virus.

  2. Icosahedral: An icosahedron is a geometric shape with 12 vertices and 20 faces. A regular icosahedron is the optimal way of forming a closed shell from identical subunits. It shows 2-3-5 symmetry. To retain this symmetry, capsomeres at the vertices are surrounded by five others and are called pentons, while capsomeres at the faces are surrounded by six others and are thus called hexons. Many animal viruses, such as rotavirus and adenovirus, are icosahedral.

Helical structure of tobacco mosaic virus.

Computer assisted reconstruction of a rotavirus particle.

  1. Prolate: This is an icosahedron elongated along the fivefold axis. It is composed of a cylinder with a cap at either end. Some bacteriophages, such ϕ29 or T4, and several fungus, plant and animal viruses show prolate structures.

  2. Enveloped: This structure is observed when a virus envelops itself in a modified cell membrane from the host to gain a viral envelope made of a lipid bilayer. Most enveloped viruses require the envelope to be infective. This envelope makes the virus vulnerable to soap and alcohol. Some viruses of this type include human immunodeficiency virus (HIV) and the COVID-19-causing severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).

  3. Complex: Complex viruses possess a capsid that is neither purely helical nor purely icosahedral, and may also possess extra structures such as protein tails or a complex outer wall. For example, bacteriophage P2 has an icosahedral head, containing the nucleic acid, attached to a cylindrical tail sheath that facilitates binding of the bacteriophage to the bacterial cell.

The nucleic acid and the capsid are collectively called the nucleocapsid.

Viral Diseases

An epidemic is the rapid spread of disease to a large number of people in a given population within a short period of time. A pandemic is a worldwide epidemic. Many epidemics and pandemics were caused by viruses, such as the 1918 influenza pandemic, the HIV/AIDS global epidemic and the COVID-19 pandemic.

  • 1918 influenza pandemic

The 1918 influenza pandemic, also known as the Great Influenza epidemic or the Spanish flu, was a deadly global influenza pandemic caused by the influenza A virus subtype H1N1 (A/H1N1). The earliest documented case was March 1918 in Kansas, United States; however, recent evidence suggests that the pandemic may have started in New York City.

Towards the end of the First World War, the disease began to spread; however, reports of the outbreak in combatant countries were concealed as a way to maintain morale. In Spain, a neutral country, the spread of the disease was highly publicised, leading to the name, ‘the Spanish flu.’

The virus travelled in four waves between February 1918 and April 1920. By 1920, an estimated 500 million people were infected and 25–50 million people had died, making this the deadliest viral pandemic and the second deadliest overall (second to the Black Death).

  • HIV/AIDS global epidemic

HIV infections encompass a spectrum of conditions which may lead to acquired immunodeficiency syndrome (AIDS).

A reconstruction of its genetic history shows that the HIV/AIDS global epidemic almost certainly originated in Kinshasa, the capital of the Democratic Republic of the Congo, around 1920. AIDS was first recognized in 1981. In 1983, HIV was discovered and identified as the cause of AIDS.

There is some debate about the classification of the outbreak of the disease—by some, it is considered a global pandemic; however, the World Health Organization (WHO) considers it a ‘global epidemic.’

The disease was first reported on 5 June, 1981. The disease continues to occur, albeit less frequently. It was estimated that global incidence of HIV peaked in 1997 at 3.3 million people.

As of 2020, there are 55.9–110 million confirmed cases and 27.2–47.8 million deaths, making the HIV/AIDS global epidemic the second deadliest viral outbreak.

There is currently no cure for HIV/AIDS.

  • COVID-19 pandemic

The COVID-19 pandemic, also called the coronavirus pandemic, is a pandemic caused by SARS-CoV-2. The virus can be traced back to 17 November 2019 in Wuhan, China. It spread around the world despite a lockdown in Wuhan and neighbouring cities. On 11 March 2020, it was declared a pandemic by the WHO.

Variants of the virus began to be identified in late 2020, with the most prominent being the Alpha variant (lineage B.1.1.7), first identified in the United Kingdom, the Beta variant (lineage B.1.351), first identified in South Africa and the Delta variant (lineage B.1.617.2), first identified in India.

With 237 million confirmed cases (780 million suspected) and 4.8 million deaths worldwide (9.9–18.6 suspected) as of 9 October 2021, this is the deadliest respiratory virus pandemic since the 1918 influenza pandemic.


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