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With the recent scare of Monkeypox and the pandemic induced by the COVID-19 virus now is the time to know a tab bit about viruses. And that is where Virology comes in. Virology is considered a part of microbiology or pathology, and it deals with the study of viruses and virus-like agents but is not limited to their taxonomy, characteristics, cultivation, and genetics, as well as their relationship with their host (Mestrovic, 2021).
In the 70s, the International Committee on the taxonomy of viruses proposed the classification of viruses. The ICTV’s nomenclature introduces viruses into order, families, and genera based on genetics, morphology, chemistry, and mechanism of multiplication. ICTV classified viruses into seven orders, 96 families, and 350 genera.
The Baltimore classification system is another system that classified viruses into seven groups based on their genomes (DNA or RNA and mode of replication). This system is an alternative to ICTV nomenclature.
For many hundreds of years, people were aware of human and animal diseases that are caused by viruses, but the term virus was first introduced in the 1890s when it co-existed with cellular life. The term virus is derived from the Latin word that simply means “slimy fluid” (Rybick, 2017).
In 1884 Charles Chamberland worked with Louis Pasture in Paris on porcelain filters. Porcelain filters were used to completely remove all bacteria or other cells from a liquid suspension, and it consists of unglazed porcelain “candles” with a pore size of 0.1-1micron. A Russian botanist Dmitri Lvanoski discovered the source of tobacco mosaic disease by using Chamberland filters in 1892. At the initial stage, he thought that the disease was caused by bacteria; after this, he experimented, bypassing the liquid through a device and using extract on the plant. As a result, the plant was again infected by diseases. He concluded that there is another extremely small bacterial spore that causes this disease. Other scientists, including Beijerinck, start an investigation of the agent that causes this disease. After a few years of Lvanovski’s discovery, the word virus was introduced in 1899. Even though It cannot be seen with the naked eye but the Lvanovski was a founder of virology.
In 1989 German scientists Paul Frosch and Friedrich Loffler introduced a second virus named foot and mouth diseases, commonly found in farms and other animals. In 1989 the fact discussed in the journal of general virology article that “sterile” filtered liquid caused infection in calves and animals, but later on, they concluded that sterile liquid was not a source of infection, but an infectious agent (tiny particle) cause infection in cows and sheep. Then it was possible to vaccinate cows and sheep against disease with a prophylactic vaccine which is obtained by heating filtered vesicle extract at a high temperature to destroy its infectivity.
The first human disease originated in Africa, named yellow fever, and later on, it was spread from Aedesaegyptii to America and nearby islands by the slave trade. Yellow fever is spread through a mosquito vector, and the virus was reported by Walter Reed in 1901; after pioneering work, it was Finlay reported by Carlos in Cuba in 1881 that mosquito is the main source of transfer of deadly disease.
In general, a Virion is a small particle that cannot be seen through the naked eye but can be seen under an electron microscope. A virus contains either RNA or DNA genome surrounded by a protective covering known as a protein coat. The size of the virion ranges from 20nm for small viruses and 900nm for typical large viruses. Viruses are small obligated parasites (intracellular), but recently, giant virus species are identified named as Pithovirussibericum and Pandoravirussalinus; their size is just equal to the bacterial cell. Virions play an important role in the transfer of their DNA or RNA into the host cell
where the genome can be expressed (transcribed or translates). The viral genome (DNA or RNA) with basic protein is enclosed in an asymmetric protein structure known as a capsid. The protein that is associated with nucleic acid is called nucleoprotein, and nucleoprotein with genome forms the nucleocapsid. The interior of the capsid contains the genome and enzymes so that they have the ability to synthesize new virions. Each capsid is made up of different types of protein subunits known as capsomers (Gelderblom, 1996). There are two types of viruses:
Enveloped viruses: Those viruses that are formed from nucleic acid, capsid, and with an additional lipid bilayer that covers the capsid are known as enveloped viruses.
Non-enveloped viruses: Those viruses that are formed from nucleic acid and capsid are known as non-enveloped or naked viruses.
In bacteriophage, there is a complex structure of the virus that genome is present in polyhedral head and sheath connected to the head with tail fibers and tail pins play a role of attachment of virus with the host cell membrane.
Viruses are small, nonliving obligate parasites that cannot replicate without a host. Classification of viruses on a different basis, such as based on genetic material, number of strands, envelope, mode of replication, host, etc.
Viruses contain either RNA OR DNA as genomic material, or some other viruses contain both DNA and RNA.
The first step of the reproductive cycle of viruses is an attachment in which the surface protein of the virus is attached to the receptor site of the host membrane to inject its genome into the host and the genome can replicate and form new viruses.
After the attachment of the virus to the host membrane then it penetrates the cell either through translocation of virion across the host cell membrane or through the fusion of the viral envelope take place.
Uncoating is a complex process from other stages in which transcription takes place, which increases the multiplication of the virus.
Assembly is the process in which the packing of a new virion takes place in such a way the newly formed viral nucleic acid and the protein form nucleocapsid of the virus. As an increasingly large number of new viruses are in the host cell, then the cell becomes bursts due to cytolysis, and a new virus becomes released.
Viruses have a great impact on living organisms by changing their immune system and causing fatal diseases. If we compare the flu virus genome and the human genome, then the flu virus contains approximately 15,000 nucleotides, and the human genome is approximately 200,000 times longer. So concluded that viruses have to be superefficient in the process of invading and propagating in the host.
Gelderblom H. (1996). Structure and Classification of Viruses.
Carter J., Saunders V. and Venetia A. (2013). Virology: Principles and Applications.
Mestrovic T. (2021). What is virology? News Medical Life Sciences.
Rybicki ED. (2017). A short history of the discovery of Viruses. Research gate.
