Viruses are submicroscopic infectious agents with both living and nonliving characteristics. They are a small piece of genetic information (DNA or RNA) inside of a protective shell (capsid).

Viruses are found in almost every ecosystem on Earth and are the most numerous type of biological entity.

Viruses are not made up of cells, so they don’t have all the equipment that cells do to make more copies of themselves. Instead, they carry instructions with them and use a host cell’s equipment to make more copies of the virus.

Must read: Biofilm – layer of microorganisms

Etymology and Discovery of Viruses

֎ The English word “virus” comes from the Latin word vīrus, which refers to poison and other noxious liquids.

֎ Dmitri Ivanowsky (1892) was the first to discover viruses. He recognised certain microbes as causal organism of the mosaic disease of tobacco. These were found to be smaller than bacteria because they passed through bacteria-proof filters.

֎ The name virus was coined by M. W. Beijerinck(1898). He demonstrated that the extract of the infected plants of tobacco could cause infection in healthy plants and named the new pathogen “virus” and called the fluid as Contagium vivum fluidum (infectious living fluid).

֎ W.M. Stanley (1935) showed that viruses could be crystallised and crystals consist largely of proteins.

֎ The study of viruses is known as virology, a subspeciality of microbiology.

Must read: Probiotics – the class of helpful microorganisms

Characteristics of Viruses

֎ Viruses are non-cellular organisms that are characterised by having an inert crystalline structure outside the living cell (host). Thus, they are inert outside their specific host cell.

֎ Once viruses infect a cell (host), they take over the machinery of the host cell to replicate themselves. Thus, virus replicates only inside the living cells of an organism.

֎ When infected, a host cell is often forced to rapidly produce thousands of copies of the original virus.

֎ Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea.

֎ Viruses that infect only bacteria are called bacteriophages and those that infect only fungi are termed mycophages . There are even some viruses called virophages that infect other viruses.

֎ Also just called “phages,” bacteriophages are viruses that specifically infect bacteria. Scientists are studying bacteriophage therapy as a potential way to treat bacterial infections that don’t respond to antibiotics.

֎ Satellite viruses can’t reproduce without other, “helper” viruses. Most satellite viruses are found in plants.

֎ The variety of host cells that a virus can infect is called its host range. 

֎ Viruses are 100 to 1,000 times smaller than the cells in our body.

֎ Viruses typically contain a protein coat surrounding a core of infectious genetic material, that could be either RNA or DNA. No virus contains both RNA and DNA.

֎ In general, viruses that infect plants have single stranded RNA and viruses that infect animals have either single or double stranded RNA or double stranded DNA.

֎ Bacterial viruses or bacteriophages (viruses that infect the bacteria) are usually double stranded DNA viruses.

Viruses: organisms at the edge of life

Viruses are considered by some biologists to be a life form, because they carry genetic material, reproduce, and evolve through natural selection, although they lack some key characteristics, such as cell structure, that are generally considered necessary criteria for defining life. Because they possess some but not all such qualities, viruses have been described as “organisms at the edge of life” and as replicators.

Must read: H1N1- subtype of Influenza A virus

Structure of Viruses

Viruses are made up of genetic material (RNA or DNA) and a protective protein coating (capsid). The protein coat called capsid made of small subunits called capsomeres, protects the nucleic acid. These capsomeres are arranged in helical or polyhedral geometric forms.

Sometimes viruses have another layer called an envelope around the capsid. Viruses without an envelope are called “naked viruses.”

Thus, when not inside an infected cell viruses exist in the form of independent viral particles, or virions, consisting of:

(i) genetic material, i.e., long molecules of DNA or RNA that encode the structure of the proteins by which the virus acts;

(ii) a protein coat, the capsid, which surrounds and protects the genetic material; and in some cases

(iii) an outside envelope of lipids.

Shapes of viruses

The shapes of the virus particles range from simple helical and icosahedral forms to more complex structures. Most virus species have virions too small to be seen with an optical microscope and are one-hundredth the size of most bacteria.

Common types of virus shapes include:

Icosahedral or polyhedral

This is a geometric shape with many sides, similar to a soccer ball. Most viruses that infect people are icosahedral.

Helical

This virus shape looks like a cylinder. Its genetic information is coiled up like a spring inside.

Spherical

Spherical viruses are helical or polyhedral viruses that have an envelope around them. They are shaped mostly like a ball.

Complex

Complex viruses combine more than one shape. Viruses that infect bacteria have a polyhedral “head” connected to a helix “body.”

Spread of Viruses

Viruses spread in many ways. One transmission pathway is through disease-bearing organisms known as vectors: for example, viruses are often transmitted from plant to plant by insects that feed on plant sap, such as aphids; and viruses in animals can be carried by blood-sucking insects.

Many viruses spread in the air by coughing and sneezing, including influenza viruses, SARS-CoV-2, chickenpox, smallpox, and measles.

Norovirus and rotavirus, common causes of viral gastroenteritis, are transmitted by the faecal–oral route, passed by hand-to-mouth contact or in food or water.

HIV is one of several viruses transmitted through sexual contact and by exposure to infected blood.

Some Common types of Viruses and Human Diseases due to Viruses

In humans viruses cause diseases like : Common colds, flu (influenza), COVID-19, Chickenpox, Measles, HIV/AIDS, HPV/genital warts, Genital herpes (HSV), Polio, Rabies, Mpox, Zika, etc.

Some common types of viruses that we might hear about include:

Influenza viruses (Orthomyxoviridae)

The Orthomyxoviridae family of viruses includes influenza A and B, which cause the flu. Strains of influenza A also cause avian flu (“bird flu”) and swine flu (H1N1).

Human herpesvirus (Herpesviridae)

Herpesviridae is a large family of viruses. They cause several types of illnesses, like oral and genital herpes, chickenpox, shingles, Epstein-Barr and cytomegalovirus (CMV).

Coronaviruses

Coronaviruses are a subfamily of viruses. SARS-CoV-2, the virus that causes COVID-19, is probably the most well-known coronavirus. But other types of coronaviruses cause mild illnesses, like a cold.

Human papillomavirus (HPV)

Human papillomaviruses are part of the Papillomaviridae family of viruses. They cause warts. Some types of HPV can lead to cancers.

Enteroviruses

Enterovirus is a genus (one level smaller than the group called a “family”) of viruses that infect your intestinal tract. Types of enteroviruses cause polio and hand, foot and mouth disease.

Flaviviruses

Viruses in this genus are often spread by mosquitoes. They cause illnesses like Zika, West Nile, dengue fever and yellow fever.

Orthopoxviruses

Viruses in the genus Orthopoxvirus cause blistering rashes. Mpox and smallpox are orthopoxviruses.

Hepatitis viruses

Though they don’t all belong to the same family or genus, hepatitis viruses all infect your liver. Hepatitis A, B and C are the most common.

Retroviruses

Retroviruses are RNA viruses that use special proteins to make DNA. The virus then inserts its DNA into yours. Your cells read the viral DNA as if it were its own instructions. HIV and human T-lymphotropic virus 1 (HTLV-1) are retroviruses.

Oncoviruses

Oncoviruses are viruses that can cause cancer. Viruses that have been linked to specific cancers include:

HPV, Epstein-Barr, HIV, Hepatitis B and C, HTLV-1 and Human herpesvirus 8 (HHV-8)

Immune responses against viruses

Viral infections in animals provoke an immune response that usually eliminates the infecting virus. Immune responses can also be produced by vaccines, which confer an artificially acquired immunity to the specific viral infection.

Some viruses, including those that cause HIV/AIDS, HPV infection, and viral hepatitis, evade these immune responses and result in chronic infections.

Some Common Viral Diseases of Plants

In plants, the symptoms produced due to infections by viruses can be mosaic formation, leaf rolling and curling, yellowing and vein clearing, dwarfing and stunted growth.

Some common viral diseases of plants are:

Tobacco mosaic, Cucumber mosaic, Barley yellow dwarf, Tomato spotted wilt, Prunusnecrotic ring
spot, Potato spindle tuber, Citrus exocortis, Yellow leaf curl of Tomato, etc.

Role of Viruses in Aquatic Ecosystems

Viruses are the most abundant biological entity in aquatic environments. There are about ten million of them in a teaspoon of seawater. Most of these viruses are bacteriophages infecting heterotrophic bacteria and cyanophages infecting cyanobacteria and they are essential to the regulation of saltwater and freshwater ecosystems.

Bacteriophages are harmless to plants and animals, and are essential to the regulation of marine and freshwater ecosystems, are important mortality agents of phytoplankton, the base of the foodchain in aquatic environments.

Bacteriophages infect and destroy bacteria in aquatic microbial communities, and are one of the most important mechanisms of recycling carbon and nutrient cycling in marine environments. The organic molecules released from the dead bacterial cells stimulate fresh bacterial and algal growth, in a process known as the viral shunt. In particular, lysis of bacteria by viruses has been shown to enhance nitrogen cycling and stimulate phytoplankton growth.

Microorganisms constitute more than 90% of the biomass in the sea. It is estimated that viruses kill approximately 20% of this biomass each day and that there are 10 to 15 times as many viruses in the oceans as there are bacteria and archaea.

Viruses are also major agents responsible for the destruction of phytoplankton including harmful algal blooms. The number of viruses in the oceans decreases further offshore and deeper into the water, where there are fewer host organisms.

Applications of Viruses

Molecular and Cell biology

Viruses are important to the study of molecular and cell biology as they provide simple systems that can be used to manipulate and investigate the functions of cells.  For example, viruses have been useful in the study of genetics and helped our understanding of the basic mechanisms of molecular genetics, such as DNA replication, transcription, RNA processing, translation, protein transport, and immunology.

Geneticists often use viruses as vectors to introduce genes into cells that they are studying. This is useful for making the cell produce a foreign substance, or to study the effect of introducing a new gene into the genome.

Medicine

Virotherapy uses viruses as vectors to treat various diseases, as they can specifically target cells and DNA. It shows promising use in the treatment of cancer and in gene therapy.

Materials science and nanotechnology

From the viewpoint of a materials scientist, viruses can be regarded as organic nanoparticles. The powerful techniques developed by life sciences are becoming the basis of engineering approaches towards nanomaterials, opening a wide range of applications far beyond biology and medicine.

Because of their size, shape, and well-defined chemical structures, viruses have been used as templates for organising materials on the nanoscale.

Synthetic viruses

Many viruses can be synthesised de novo (“from scratch”). The first synthetic virus was created in 2002. This technology is now being used to investigate novel vaccine strategies.

Biological warfare

The ability of viruses to cause devastating epidemics in human societies has led to the concern that viruses could be weaponised for biological warfare. Further concern was raised by the successful recreation of the infamous 1918 influenza virus in a laboratory.

PRACTICE QUESTIONS

QUES . Consider the following statements: UPSC PRELIMS 2025

I. No virus can survive in ocean waters.

II. No virus can infect bacteria.

III. No virus can change the cellular transcriptional activity in host cells.

How many of the statements given above are correct?

(a) Only one

(b) Only two

(c) All the three

(d) None

Show Answer