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Virology is simply defined as the study of viruses. Medical virology studies viruses that are of medical importance i.e., those groups or families of viruses that cause disease in humans. These groups of viruses that cause disease in humans and/or animals as well as in plants are known as pathogenic viruses. Virology therefore is the study of viruses inclusive of those ones that are of medical importance and those that are not harmful but are beneficial in nature, especially for vaccine production and for the production of other biopharmaceuticals. In the field of medical virology, pathogenic viruses that cause diseases in humans are isolated, characterized and identified from clinically important samples – with the sole aim of guiding therapy targeted at their treatment and possible eradication from the body of the infected human or animal host. And this is why the knowledge of virology is important. Viruses are non-cellular microorganisms that consist of nucleic acids (either DNA or RNA) which are surrounded by a protein coat known as capsid. They are particles of nucleic acid molecules, either DNA or RNA that are usually surrounded by protein molecules and lipid by lipid membranes in some virions. In other words, viruses are acellular complexes or molecules that consist only of a DNA genome or RNA genome and a protein coat that surrounds its nucleic acid molecule. They are obligate intracellular parasites that only replicate within a living host cell. Viruses are different from the other groups of microorganisms (i.e., bacteria, fungi, protozoa and algae) in several ways. For example, viruses only replicate in particular living cells including cells of other microorganisms, plants, and animals. And the nucleic acid genome of a virus is either made up of DNA or RNA and not both in the same organism or virion.


Viruses are very small in nature when compared to other microbes such as bacteria and fungi; and they are 100 times smaller than bacteria. They rarely exist or reproduce on their own; and thus viruses cannot survive outside a living cell. They are inert outside a living cell because they lack the capability for independent cell metabolism. This is why viruses require a living host cell in order to undergo the process of replication and thus carry out other vital cell functions. Viruses do not contain both DNA and RNA in one organism as their genome but rather, each virion either contains DNA or RNA as its nucleic acid and not both in one organism. For example, the genome and/or nucleic acid molecules of a bacterium are made up of both RNA and DNA – which are present in the same organism. But this is not the case for a virus – which only contain one type of a particular nucleic acid molecule (DNA or RNA) at any given time in a given virion. No virus contains both DNA and RNA in the same organism, as is applicable in other microbial life forms such as bacteria, algae, protozoa and fungi. A virion (Plural: virions) is a single complete virus particle. Virion is a term that is used to describe a matured virus particle or virus; and it is a synonym for virus particle. Virions are formed in vivo inside the infected host cell when the invading viral particle takes over the host cell’s machinery and makes it to produce the components of new viruses (such as proteins) that have the same genetic makeup as the parent viral cell. Virion consists mainly of three parts: 1) a nucleic acid genome (DNA or RNA); 2) a capsid or protein coat; and 3) envelop (that surrounds the capsid) which is found in some viruses (known as enveloped viruses). The general structure of a virion or viral particle is shown in Figure 1. Another word for capsid is nucleocapsid. Nucleocapsid is a combination of the genome and capsid. Viruses that contain envelops in addition to the usual capsid and genome are generally known as complex (enveloped) viruses while those that contain only the capsid and genome are termed naked (simple) viruses (Figure 1).

Figure 1. General structure of a virus. A = Naked virus. B = Enveloped virus. Photo courtesy:

Microbiologists especially virologists should always have knowledge about how pathogenic virus enters a host, spread and cause disease because this is important for the proper laboratory and/or clinical diagnosis, effective treatment and for the control and prevention of the spread of viral infection in either human or animal populations. Scientists or microbiologists that study viruses, and whose area of specialization is on viruses (inclusive of animal, plant and human viruses) are known as virologists. Virologists can work in the hospital, government/private institutions, livestock/poultry production facilities, veterinary institutions and even in agricultural sectors – where they harness their knowledge about viruses to contain the nefarious activities of pathogenic viruses. Capsid (protein coat), a nucleic acid genome (DNA or RNA) and viral envelop are the main components of a viral particle or virion. The capsid or protein coat helps to protect the viral nucleic acid genome from destruction or inactivation by nucleases. Nucleases are enzymes that hydrolyze the phosphodiester bonds of DNA and RNA structure; and they include ribonucleases and deoxyribonucleases. While ribonucleases act only upon ribonucleic acids (RNA), deoxyribonucleases only act upon deoxyribonucleic acids (DNA). The capsid also facilitates the attachment of viruses to host cells as well as their transfer from one cell to another. The nucleic acid genome (comprising RNA or DNA)contains the genetic information required to produce viral proteins and other molecules necessary for the formation and coupling of new virions. The envelope surrounds the nucleocapsid of some viruses; and they bear specific viral glycoproteins that facilitate viral attachment to host cell(s) during infection. As shown in this illustration (Figure 1), the capsid, nucleic acid genome and envelope are the main components of a virus. The nucleocapsid is a combination of the capsid and nucleic acid genome of a virion. While naked viruses lack envelope (an external structure outside the nucleocapsid), enveloped viruses have envelopes which protects or surrounds its nucleocapsid. 

Viruses are ubiquitous in the universe. They play various economic roles ranging from causing epidemics and other viral infectious diseases in human, animal and plant populations, to their utilization in molecular medicine and pharmaceutical industry for the production of effective vaccines for disease prevention and control. Viruses are distinctive from other microbial cells because they infect other forms of life including prokaryotic and eukaryotic cells. Bacteriophages are viruses that infect bacterial cells or prokaryotes. Such group of viruses that infect bacteria can also be known as phages. Bacteriophages are prevalent in nature; and when they attack bacteria especially a bacterial culture, there is an observable appearance of cleared zones known as plaques that shows the points where the bacterial cells growing in the culture plate have been lysed. It is noteworthy that viruses are not actually cells per se; and this is due in part that they solely depend on their host cell for their own cellular and/or metabolic activities. However, viruses are not completely regarded as non-living entities because they also exhibit some level of life as other forms of microorganisms. For example, viruses have their own genome (DNA or RNA) like other living organisms. They are capable of autonomous replication but only with the support of the host cell’s genome which they infect and overpower. Viruses are adaptable to a particular niche or habitat especially those of a living cell that they parasitize in order to assume a living state. Nonetheless, viruses exist as inert cells outside of a living host cells; and they are generally maintained in the laboratory in cell/tissue cultures in vitro and even in egg embryos in vivo.

Viruses possess no functional organelles for the synthesis of their own macromolecules (e.g., proteins) as is obtainable in other forms of microbial life including bacteria and fungi. They are metabolically inactive or inert outside their host cell; and in such scenarios, viruses only exist as crystallized forms from which they can easily be transmitted to susceptible cells to cause infection. Since they lack their own independent cellular, metabolic and reproductive prowess; viruses take over the cellular machinery of the cells which they infect and cause same to synthesize products that are beneficial to them (i.e., the invading viral particle). Within the infected host cells, viruses mainly exist as replicating nucleic acid particles (DNA or RNA) coated with protein coat; and they stimulate and overwhelm the host’s metabolic and biosynthesizing machinery to produce the cellular components required for the formation of complete and new virions. Viruses also exist in the soil, water and air. Viruses like other microbes are ubiquitous and they are found in the air (for example, influenzae virus that cause flu in humans), in water and food (for example, rotavirus that cause gastroenteritis in humans); and they can be transmitted to humans through vectors (for example, yellow fever virus transmitted to humans through the bite of some mosquitoes such as Aedes mosquitoes); through sexual intercourse (for example, human immunodeficiency virus, HIV); and through direct body contact with other infectious materials, infected humans or animal wastes (for example, Ebola virus and Lassa fever virus transmitted to humans through contact with the natural hosts of the virus).

The study of virology is significant now than ever before, owing to the fact that some viral particles or viral diseases are now emerging and re-emerging. Several outbreaks of viral diseases including but not limited to Ebola haemorrhagic fever, chikungunya, zika and Lassa fever virus infections have been recently reported in some parts of the world. AIDS, severe acute respiratory syndrome (SARS), influenza virus infection, rabies, respiratory synctial virus (a common cause of pneumonia in infants), dengue fever, human Papilloma virus (causative agent of cervical cancer in women) and hepatitis infection (especially hepatitis B virus, HBV and hepatitis C infection) are some viral infections that still parasitize or harm humanity. Most of these viral infections or viral infectious particles have continued to cause several public health issues across the world, thus putting the public health into jeopardy. These infectious diseases caused by pathogenic viruses and many other deleterious effects of these unique class of microorganisms (i.e., the pathogenic viruses) have made the study of viruses more significant than ever before. And it is critical that the microbiologists and other medical and biomedical scientists become well-informed about the basic knowledge of viruses and how these small infectious diseases particles could be properly contained and harnessed for the benefit of all mankind. Only a proper knowledge of viruses can help health care practitioners, researchers and the academia to develop ways of containing viral infections especially now that some recent viral disease outbreak (for example, Ebola virus disease, EVD) has continued to remind us that pathogenic virus or viral infections is still on the horizon.

Further reading

Acheson N.H (2011). Fundamentals of Molecular Virology. Second edition. John Wiley and Sons Limited, West Sussex, United Kingdom.

Brian W.J Mahy (2001). A Dictionary of Virology. Third edition. Academic Press, California, USA.

Cann A.J (2011). Principles of Molecular Virology. Fifth edition. Academic Press, San Diego, United States.

Carter J and Saunders V (2013). Virology: Principles and Applications. Second edition. Wiley-Blackwell, New Jersey, United States.

Dimmock N (2015). Introduction to Modern Virology. Seventh edition. Wiley-Blackwell, New Jersey, United States.

Kudesia G and Wreghitt T (2009). Clinical and Diagnostic Virology. Cambridge University Press, New York, USA. 

Marty A.M, Jahrling P.B and Geisbert T.W (2006). Viral hemorrhagic fevers. Clin Lab Med, 26(2):345–386.

Strauss J.H and Straus E.G (2008). Viruses and Human Diseases. 2nd edition. Elsevier Academic Press Publications, Oxford, UK.

Zuckerman A.J, Banatvala J.E, Schoub B.D, Grifiths P.D and Mortimer P (2009). Principles and Practice of Clinical Virology. Sixth edition. John Wiley and Sons Ltd Publication, UK.

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