What is a virus?

Viruses are infectious microscopic agents that can only reproduce in live cells of animals, plants, or microbes and are smaller than bacteria. Vertebrates, invertebrates, plants, protists, and fungi are among their many hosts. Viruses are unable to multiply on their own due to the lack of necessary cellular machinery, but they can guide the cell machinery of a host to make additional viruses by infection. The virus enters and attaches to a particular host cell, injects its genetic material, and reproduces by using the host’s genetic material. Viruses cause infection by taking entry into the host cell and using the host cell’s enzymes and raw materials to multiply.

The structure of viruses

They are extremely small, sizes ranging from 20 to 400 nanometers. Viruses have either ribonucleic acid (RNA), or deoxyribonucleic acid (DNA) as their genetic material, which is found within the protein coat called the capsid. The nucleocapsid is the term given to the capsid (protein coat) and the nucleic acid it encloses. The nucleic acid can be single-stranded or double-stranded. The virus particles that cause infections contain nucleic acid surrounded by a capsid and are called virions. Simple helical and icosahedral shapes, as well as more complicated constructions, are possible. The protein coat is composed of numerous copies of protomer proteins encoded by a single viral gene. The spikes on the virus surface assist the virus in attaching to the host cell.  

The classification of viruses

Viruses are categorized according to their size and form, chemical makeup and genome structure, reproduction mechanism, and kind of host. Human-infecting viruses are currently divided into 26 families, which represent only a part of the vast number of viruses whose host range includes vertebrates to protozoa, as well as plants, bacteria, and fungi.

Viruses are categorized atlevel of order, family, subfamily, genus, and species in the hierarchical classification system based on shared features. Over 30,000 distinct viral isolates are classified into 3,600 species, 71 families, and 164 genera.

Classification based on the shape of the capsid

Based on the capsid structure, viruses can be classified as helical symmetry and icosahedral symmetry. In viruses with helical symmetry, protomers arrange to form a helical-shaped capsid that encloses the genetic material. Tobacco mosaic virus (TMV) is the well-studied helical virus (arranged on a helix of pitch 2.3 nm with 16.33 protein subunits per helical turn and 300 nm long and 18 nm in diameter).

In viruses with icosahedral symmetry, the protomers, or chains of the structural polypeptide, are grouped into clusters of oligomers called capsomeres, which create a closed-form of the capsid. Papillomavirus (72 pentameric capsomeres) is an icosahedral virus.

Classification based on the type of nucleic acid

Viruses can be categorized into DNA viruses and RNA viruses based on the type of nucleic acid. DNA viruses are viruses whose genetic material contains DNA (deoxyribonucleic acid). Parvovirus, herpesvirus, and papillomavirus are DNA viruses. These viruses can harm both animals and humans, and their symptoms range from mild to life-threatening. There are two types of DNA viruses: double-stranded and single-stranded. The former has two-stranded DNA and the latter has DNA with only one strand. Poxviruses, adenoviruses, bacteriophages T2, T3, T4, T7, T6, and herpes viruses are double-stranded DNA viruses. Bacteriophages X and 74 are viruses with a single DNA strand.

RNA viruses are viruses whose genetic material is RNA. Rotavirus, poliovirus, measles virus, yellow fever virus, rabies virus, dengue virus, Ebola virus, and influenza virus are among them. There are two types of RNA viruses; double-stranded RNA viruses and viruses with single-stranded RNA. The former is found in the capsids of reoviruses in mammals and rice dwarf virus and wound tumor virus in plants. Tobacco mosaic virus, bacteriophage MS-2, poliomyelitis, influenza virus, and avian leukemia virus are among the latter.

Classification based on the type of host

The viruses can be categorized into groups according to the host they infect. They are animal viruses, insect viruses, plant viruses, and bacteriophages. Animal viruses infect different types of animals. They include severe acute respiratory syndrome coronavirus 2 (SARS-CoV2), rabies, influenza, mumps, and poliovirus. Plant viruses infect by invading the plant’s cells. Tobacco mosaic virus, turnip yellow virus, potato virus, and beet yellow virus are plant viruses. Bacteriophage viruses invade bacterial cells. Bacteriophages are available in different forms and sizes. Typically, each bacteriophage will only attack one strain of bacteria. Viruses like DNA viruses, MV-11, RNA viruses, and lambda phage are bacteriophages. Baculoviruses (BVs), granuloviruses (GVs), ascoviruses, parvoviruses, and poxviruses are insect viruses.

Types of viruses that infect humans

  • Chikungunya virus infects humans, monkeys, and mosquitoes. They enter the host through animal and arthropod bites and causes fever and joint pain.
  • Dengue virus: The virus infects the hosts, humans and mosquitoes, through animal and arthropod bites. It causes hemorrhagic fever.
  • Ebola virus is transmitted to the hosts-humans, monkeys, and bats-through animal bites and contact and causes hemorrhagic fever disease.
  • Epstein-Barr virus infects humans via contact and saliva and causes mononucleosis.
  • Hepatitis A virus infects humans via fecal-oral transmission. The virus spreads the hepatitis infection.
  • Hepatitis B virus infects humans and chimpanzees through blood and sexual contact and spreads hepatitis.
  • Human immunodeficiency virus (HIV) is transferred to humans via transmission of bodily fluids, such as during blood transfusion, sexual contact. It results in AIDS (acquired immunodeficiency syndrome).
  • Influenza A virus infects humans, birds, and pigs via respiratory routes and animal contact. It causes flu.
  • Poliovirus infects humans and mammals via fecal-oral transmission. It causes the disease poliomyelitis.
  • Zika virus infects humans, monkeys, and mosquitoes through animal and arthropod bites and causes fever, joint pain, and rash.
  • Human coronaviruses infect humans, bats, and palm civets by contact and respiratory transmission. 229E, NL63, HKU1 and OC43 are the common human coronaviruses. Other examples include MERS-CoV (Middle East Respiratory Syndrome), SARS-CoV (severe acute respiratory syndrome), and SARS-CoV-2 (the novel coronavirus that is the pathogen of coronavirus disease or COVID-19). 229E, NL63, HKU1, and OC43 are regularly found in people all around the world. Coronaviruses that infect animals can sometimes mutate and make people sick and have adverse effects on health. 2019-nCoV (COVID-19), MERS-CoV and  SARS-CoV are three recent examples.

Vaccines (containing dead or attenuated pathogen) against viral diseases are available; they have saved millions of people’s health and continue to do so. Community transmission will diminish as more people get vaccinated. The vaccine's purpose is to prevent disease or illness through the formation of memory cells that are activated upon infection and neutralize the pathogen before the symptoms are developed. In a sense, the vaccine must be classified as a healthcare service.

The image shows the modes of transmission

Context and Applications

This topic is significant in the biotechnology, medicine, and the exams of school, graduate, and post-graduate levels, especially for Bachelor's of Science in Biology and Biotechnology and Masters of Science in Biology and Biotechnology.

Practice Problems

Question 1: The virus genetic material is encased in a protein coat known as?

A) Capsid

B) Nucleocapsid

C) Virion

D) None

Answer: Option A is correct.

Explanation: Viruses have nucleic acid as their genetic material, and they are coated with a protein coat known as a capsid.

Question 2: Which of the following regarding a virion is correct?

A) The virus capsid

B) A fully developed and infectious viral particle.

C) Viral genetic material

D) None

Answer: Option B is correct.

Explanation: The whole infectious virus particle, which consists of nucleic acid and an outer protein shell is known as a virion.

Question 3: An example for helical virus

A) Poxvirus

B) Herpesvirus

C) TMV -Tobacco mosaic virus


Answer: Option C is correct.

Explanation: The tobacco mosaic virus is a well-studied helical virus. Poxvirus is oval-shaped. Herpesvirus is icosahedra-shape. COVID-19 is crown-like.

Question 4: Which of the virus given below is transmitted by fecal-oral transmission?

A) Human immunodeficiency virus

B) Hepatitis A virus

C) Influenza A virus

D) None of the above

Answer: Option B is correct.

Explanation: HIV is transmitted through sexual contact. The influenza virus is transmitted by respiratory or zoonosis way.

Question 5: Which one of the following is a bacteriophage?

A) Baculoviruses

B) MV-11

C) 229E

D) None of the above

Answer: Option B is correct.

Explanation: DNA viruses, MV-11, RNA viruses, λ page, etc., are bacteriophages. Baculoviruses are insect viruses. 229E is a human coronavirus (animal virus).

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