Viruses- Introduction, History, Features, Classification, Diseases


A virus is a microscopic infectious agent that can cause diseases which is able to utilize the synthetic machinery of the living cell of another organism for its multiplication. Virus are the smallest living organism ranging from about 20nm to 300nm that are about 50 times smaller than bacteria approximately.  They are too small to be seen with the light microscope so that they are observe under the electron microscope.

Virus are unique organism in many ways:

  • They cannot be termed as prokaryotes or eukaryotes as they do not have a cellular structure.  They contain hereditary material in the form of DNA por RNA enclosed in protein coat.
  • They exhibit properties of both living and non-living things. A virus has no metabolic activity of its own. It becomes active and multiples when it infects (attacks) a specific living host cell. When removed from a cell, it can be stored as a crystal in a bottle for several years. A virus can survive as a parasite only inside a living cell.
  • Virus are considered to be smallest living organism. The biologists are not, clear so far whether they are early primitive forms of living things or a highly evolved super parasite.
  • Virus do not contain both nucleic acids (DNA and RNA). Only one of them is present in it. It consists of genetic material, either DNA or RNA, surrounded by a protein coat.

Non-living properties of Virus:

  • Virus lack organelles characteristics of cell.
  • They depend on their hosts for their survival and multiplication.
  • They are inactive outside of living hosts.
  • Unlike cellular organism, Virus cannot reproduce by fission.
  • Multiplication process (replication) of virus is entirely different from reproduction of living organisms.
  • Some virus can be crystallised, e.g., Tobacco mosaic virus and coxsackievirus
  • Virus don’t spire.

Living properties of Virus:

  • They are particulate and strictly obligate intercellular parasites.
  •  They can grow inside the host cell and are capable of self-reproduction while inside host cell.
  • Virus c like cells, can mutate and change characteristics
  • They occur in definite races, or strains each with its specific character
  • They contain either DNA or RNA.
  • They respond to heat, chemical and radiation
  • They are transmitted form the diseases host to the healthy ones.

Historical Background:

The study of viruses and their discovery have progressed over the years. Researchers like Dmitry Ivanovsky and others noted that certain diseases were caused by infectious agents smaller than bacteria in the late 19th century. Ivanovsky’s discovery of the tobacco mosaic virus and Frederick Twort’s and Félix d’Hérelle’s subsequent identification of bacteriophages were major turning points in the study of viruses.

In the 1930s, the discovery of electron microscopy allowed for direct observation of viruses, leading to additional improvements to virology. The branches of science which deals with the study of virus is called virology.

The mid-twentieth century witnessed the classification of viruses into DNA and RNA viruses, followed by the discovery of various human viral illnesses. Modern molecular biology tools have been significant for identifying novel viruses, determining their genetic makeup, and developing antiviral therapies. The continual study of new viruses, as well as the ongoing drive to treat viral infections, ensuring that the study of viruses remains a vibrant and crucial subject of scientific research.


The structure of a virus is relatively simple. Typically, a virus is made up of two components. – nucleoid/nucleic acid and protein coat called capsid. Some viruses may also have an outer envelope composed of lipids.  Nucleoid is also called core. It is made up of a strand of nucleic acid. Nucleic acid is either DNA or RNA. DNA containing virus are called deoxyvirus while RNA containing virus are called ribovirus. It can be single-stranded or double-stranded, depending on the type of virus. The genetic material carries the genetic code that determines the viral characteristics and can encode the necessary proteins for its replication.

Basic structure of virus

Fig: Basic structure of virus

The genetic material is encased in a protective protein shell called a capsid. The capsomeres, which are repeating protein components, combine to form the overall structure of the capsid. The capsid is protective in function and have enzymatic properties. These proteins are resistant to the proteolytic enzymes of the host. The arrangement of capsomeres can vary, resulting in diverse form of the virus particle such as helical, icosahedral, or complicated architectures.


Viruses are classified in various ways which include based on their genetic material, structure, mode of replication, and the type of host they infect.

Genetic Material

  • DNA viruses: These viruses which contain DNA as their genetic material is called DNA virus. Examples include bacteriophage, animal virus, herpesviruses, adenoviruses, and poxviruses.
  • RNA viruses: These viruses have RNA as their genetic material is called RNA virus. Examples include influenza viruses, HIV, and coronaviruses.
  • DNA-RNA virus: Some virus contains both DNA and RNA is called DNA-RNA virus such as Leuko virus, RNA tumour virus.

Host Specificity

  • Animal viruses: These viruses infect animals, including humans. Examples include influenza viruses, Polio virus, rabies virus.
  • Plant viruses: These viruses infect plants and can cause diseases in crops. Examples include TMV (Tobacco Mosaic Virus), PV (Potato Virus), TYV (Turnip Yellow Virus).
  • Bacterial viruses (Bacteriophages): The viruses infect bacteria are called bacteriophage.
  • Mycophage: Virus that infects fungi
  • Phycophage: virus that infects algae
  • Cyanophage: Virus that infects blue green algae/cyanobacteria
  • Zymophage:  Virus that infects yeasts


  • Enveloped viruses: These viruses have a lipid envelope surrounding their genetic material. E.g., influenza viruses, HIV, and herpesviruses.
  • Non-enveloped viruses: These viruses lack a lipid envelope. E.g., adenoviruses and noroviruses.

Mode of Replication

  • Retroviruses: These viruses use reverse transcription in which RNA genome is converted into DNA, which is then finally integrated into the host cell’s genome. E.g., HIV
  • Lytic viruses: These viruses replicate inside the host cell, causing its destruction upon release of new viral particles. Bacteriophages are examples of lytic viruses.
  • Latent viruses: These viruses can remain dormant inside the host cell for extended periods without causing significant harm. Herpesviruses are examples of latent viruses.

Mode of transmission

Blood-Borne Viruses (BBVs)

Some persons contain viruses in their blood, which can be spread by contact and are referred to as blood borne viruses. E.g., Human Immunodeficiency Virus (HIV), Hepatitis B virus (HBV), and Hepatitis C virus (HCV).

Respiratory Viruses

Respiratory viruses are viruses that affect the nose, throat, and breathing passages, including the lungs, and are spread when an infected person coughs, sneezes, talks, or exhales through respiratory droplets. E.g.,   Influenza viruses, Rhinoviruses (cause of the common cold), and Coronaviruses (including SARS-CoV-2).

Faecal-Oral Viruses

These viruses are commonly found in contaminated food, water, or poor hygiene practices and are transmitted through an infected person’s faeces E.g.,   Noroviruses (cause of gastroenteritis), Hepatitis A virus, and Rotaviruses (cause of severe diarrhoea in children), E. coli, adenovirus

Vector-Borne Viruses (VBDs)

Vectors such as mosquitoes, ticks, and other arthropods spread these viruses that helps as an intermediary host, transporting the virus from one host to the next. E.g.,   Dengue virus, Zika virus, West Nile virus, zika virus (ZIKV), Yellow fever virus (YFV)

Sexual Transmission Viruses

These viruses are primarily transmitted through sexual contact. E.g.,   Human Papillomavirus (HPV), Herpes Simplex Virus (HSV), and Human Immunodeficiency Virus (HIV).

Direct Contact Viruses

Direct contact with infected people or their bodily fluids, such as saliva or skin-to-skin contact, can spread these viruses. E.g.,   Herpes Simplex Virus (HSV), Varicella-zoster virus (causes chickenpox and shingles), and Human Papillomavirus (HPV).

Baltimore classification system:

The Baltimore classification system was developed by David Baltimore in 1971 and is a popular methodology for classifying viruses according to their genome structures and replication techniques. It categorizes viruses into seven groups. This classification approach offers important insights into viral biology and serves as a basis for comprehending their various replication mechanisms.

The Baltimore classification of virus

Fig: The Baltimore classification of virus

Common Viral Diseases:

  • Influenza
  • COVID-19
  • Measles
  • Ebola
  • Mumps
  • Measles
  • Viral Hepatitis

Prevention and Control of Viral Infections:

Virus infection prevention and control are crucial to protecting public health and reducing the effect of these diseases. Widespread vaccination programs to establish immunity, basic hand hygiene practices, covering the mouth and nose when coughing or sneezing, and utilizing personal protective equipment in hospital settings are all effective tactics. Furthermore, keeping clean settings through frequent disinfection, avoiding close contact with infected individuals, practicing safe food handling, implementing vector control measures, and enforcing public health measures all help to reduce virus transmission. To ensure that the most suitable and effective steps are taken in each specific scenario, it is critical to follow the instructions of local health authorities.


  • Murray, P.R., Rosenthal, K.S. and Pfaller, M.A., 2020. Medical microbiology E-book. Elsevier Health Sciences.
  • Prescott, L.M., 2002. Microbiology.

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