Nomenclature and Classification of Virus: Definition, History, Structure, Classification

Q: How are virus names written?

Since 1966, the International Committee on Taxonomy of Viruses (ICTV) has systematically organised the classification and naming (nomenclature) of viruses at higher taxonomic levels like families and genera.

Nomenclature and Classification of Virus: Definition, History, Structure, Classification

Edited By Irshad Anwar | Updated on Jul 02, 2025 06:04 PM IST

A virus is a microscopic infectious agent that can only replicate inside the living cells of a host organism. They are composed of genetic material (DNA or RNA) enclosed in a protein coat. Viruses lack cellular structure and metabolism, making them dependent on host cells for reproduction. Virus Nomenclature and Classification involve naming viruses based on their genetic material, structure, host, and mode of replication.

This Story also Contains

What Are Viruses?
History Of Virus Classification
Principles Of Virus Classification
Virus Nomenclature
Categories Of Viruses
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Nomenclature and Classification of Virus: Definition, History, Structure, Classification

Some viruses may have evolved from plasmids, which are pieces of DNA that can move between cells. Other viruses may have evolved from bacteria. This article includes learning about the viruses, the History Of Virus Classification and Virus Nomenclature. Virus is a topic of the chapter Biological Classification in Biology.

What Are Viruses?

Viruses can be defined as tiny infectious microorganisms that can replicate themselves only in the living cells of a host organism. The virus classification divides viruses according to the genetic material and the morphology of the virus particle, the method of replication, and the range of hosts that can be affected. This gives clues regarding the virus's evolution and the manner of infection.

Viruses are named depending on the species of the host, the form of the disease that is caused, the geographical origin of the virus, or the molecular nature of the virus, which enhances inter-generational understanding between scientists. Such a systematic approach not only assists scientific knowledge of diseases and diagnosis but also helps in policy formulation on the control and prevention of viral diseases in different parts of the globe.

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History Of Virus Classification

The history of virus classification has been characterised by certain landmarks and outstanding scientists who have contributed to the study of these mysterious entities.

Early efforts- 18th century

Classification done by Carl Linnaeus laid the basis of the taxonomical approach of categorising organisms where viruses also fell under this channel of thinking.

The advancements in the 20th century

The instrument electron microscopy helped to define the ultrastructure of viruses, as well as virus taxonomy based on the virus morphology.

Modern Era - 1950s

The researcher David Baltimore helped in categorising viruses according to the genetic material, i.e., DNA or RNA, with their modes of replication inside host cells. It introduced the Baltimore classification of seven groups of viruses based on RNA genome replication strategies, changed the approach to the study of viruses, and established a basis for the classification and evolution of viruses.

These landmarks and scientific achievements remain an important foundation for virus classification and make it possible to update the concept of viruses, anticipate the risk of virus-related diseases, and facilitate the search for effective approaches to intervening in viruses at the population level and in medicine.

Principles Of Virus Classification

Virus classification helps scientists organize viruses based on their similarities, structure, genetic material, and replication methods. This makes it easier to study viruses, understand how they cause diseases, and find ways to treat or prevent them. They are not classified under any of the five kingdom classification of diversity because they exhibit characteristics of both living and non-living things.

Taxonomic Hierarchy (Linnaean System)

They are categorised into classification levels as Domain Kingdom, Phylum, Class, Order, Family, Genus and Species. This classification helps in categorising viruses in a manner that they share one or more features with other viruses and or evolved from one source.

ICTV (International Committee On Taxonomy Of Viruses)

The ICTV is the major global organisation that supervises the classification and naming of viruses to ensure some form of order and uniformity in the virus classification system in the world. It is rather invaluable in the effort to maintain consistency in the approach towards the categorisation of viruses in the system that informs the systematic consideration and description of viruses by researchers, clinicians, public health officials, and others.

Baltimore Classification

The classification of viruses is placed into a taxonomy system that allows their integration into a genetic and replicative framework that is virology. It categorises viruses based on 7 classes (I-VII), which are marked by high similarity in the process of mRNA synthesis. There is nothing more important than the Baltimore classification when it comes to describing how viruses reproduce themselves and if there can be a method for combating distinct classes of viruses.

Virus Nomenclature

Naming viruses properly is very important in virology to avoid confusion and ensure clear communication among scientists, doctors, and public health officials. Virus names are often based on their structure, the diseases they cause, or where they were discovered.

Binomial Nomenclature In Viruses

The classification of viruses according to their type is done in a binomial way with the name of the virus being of the format genus and species.

Format: Genus + species

For example, the Influenza virus, which is categorised under Influenzavirus A

How Virus Names Are Assigned

There are international committees comprising experts in virus taxonomy, such as the International Committee on Taxonomy of Viruses (ICTV), responsible for guiding how viruses are classified and named to avoid confusion and complex discussions in scientific forums.

Basis of Name	Example
Geographic location	Ebola virus (named after the Ebola River)
Disease caused	Hepatitis viruses (cause liver disease)
Scientist's name	Epstein-Barr virus (named after the discoverers)

Some virus names are written as acronyms:

HIV - Human Immunodeficiency Virus
HBV - Hepatitis B Virus
ZIKV - Zika Virus

Categories Of Viruses

The viruses are mainly classified based on the type of genetic material (DNA or RNA) inside them and how that functions inside a host cell. These characteristics determine how the virus replicates and causes disease.

DNA Viruses Vs. RNA Viruses

The key difference between DNA and RNA are given as below:

DNA viruses like Herpes simplex virus & Variola virus possess double-stranded DNA genomes along with all types of viruses.
RNA viruses consist of Single-stranded RNA, with Hepatitis C as an example, and single-stranded RNA Influenza virus.

Single-Stranded Vs. Double-Stranded Viruses

There are different types of viruses depending on how many strands of nucleic acids they contain which can be

Single-stranded (ss) - Ex Hepatitis C virus
Double-stranded (ds) - Ex Rotaviruses

Positive-Sense Vs. Negative-Sense RNA Viruses

The RNA viruses are further classified into

Positive-sense RNA viruses: +ssRNA (for example, Poliovirus) - RNA can directly act as mRNA and make proteins.
Negative-sense RNA viruses: −ssRNA (for example, Measles virus) - RNA cannot directly make proteins; it needs to be converted to +RNA by RNA polymerase.

Also Read:

Difference Between Virus And Bacteria	Virus Definition
Prions	Viroids
Bacterial Viruses	Lichens

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Frequently Asked Questions (FAQs)

1. What is the nomenclature of viruses?

This is the method of referring to the virus to an alphabetical/numbered name and a genus and species name by the characteristics or diseases that it causes for example Human Immunodeficiency Virus – HIV.

2. What are viruses types based on their genetic material?

DNA and RNA are the two types of viral genomes: dsDNA or ssDNA for DNA viruses, and ssRNA or dsRNA for RNA viruses, necessary for analyzing the mechanisms of viral reproduction and, therefore, creating antiviral medications.

3. What is the role of ICTV in virus taxonomy?

The International Committee on Taxonomy of Viruses (ICTV) is responsible for the global classification of viruses including coming up with rules on how viruses should be named and sorted according to their phylogenetic trees.

4. How do enveloped and non-enveloped viruses differ?

The enveloped viruses have an outer lipid membrane which is obtained from host cells; this enables them to avoid the host’s immune system and also to penetrate host cells. The viruses of this family along with Non A, and Non B hepatitis lack this membrane and therefore can be more resistant in the environment but at the same time are less resistant to disinfection.

5. How are virus names written?

Since 1966, the International Committee on Taxonomy of Viruses (ICTV) has systematically organised the classification and naming (nomenclature) of viruses at higher taxonomic levels like families and genera.

6. What is the Baltimore classification system for viruses?

The Baltimore classification system, developed by David Baltimore, categorizes viruses based on their genetic material and method of replication. It divides viruses into seven groups, from single-stranded DNA to double-stranded RNA viruses, helping to understand their basic properties and replication strategies.

7. How does the structure of a virus influence its classification?

Virus structure plays a crucial role in classification. Factors such as the type of nucleic acid (DNA or RNA), presence or absence of an envelope, capsid shape, and overall size are used to group viruses into families and genera. These structural features often correlate with the virus's behavior and host range.

8. What is a viral species and how is it defined?

A viral species is the lowest taxonomic level in virus classification. It's defined as a group of viruses sharing the same genetic material, host range, cell and tissue tropism, pathogenicity, and mode of transmission. However, defining viral species can be challenging due to their rapid evolution and genetic variability.

9. What is the significance of the order "Mononegavirales" in virus classification?

Mononegavirales is an order of viruses that includes important human pathogens like rabies and Ebola. This order is significant because it groups viruses with similar genomic structure (non-segmented, negative-sense RNA) and replication strategies, illustrating how classification can reveal important biological relationships.

10. What is the difference between a virus family and a virus genus?

A virus family is a higher taxonomic rank that groups related genera of viruses. Families often share broad characteristics like genome type and virion structure. A genus is a lower rank within a family, grouping viruses with more specific similarities in genetic sequence, host range, and replication strategy.

11. How has the discovery of giant viruses impacted virus classification?

The discovery of giant viruses, like Mimivirus, has challenged traditional views of virus classification. These viruses have genomes larger than some bacteria and possess genes typically found only in cellular organisms. This has led to debates about the definition of viruses and the need for new classification criteria to accommodate these unique entities.

12. What is the importance of studying virus evolution in classification?

Studying virus evolution is crucial for classification as it reveals the relationships between different viral groups. Evolutionary analysis helps in understanding the origin of viruses, their adaptation to new hosts, and the emergence of new strains. This information is vital for creating a classification system that reflects natural relationships and predicts viral behavior.

13. How do environmental viruses challenge traditional classification methods?

Environmental viruses, discovered through metagenomic studies, often lack culturable hosts and complete genomic sequences. This challenges traditional classification methods that rely on host range and complete genomic analysis. Scientists are developing new approaches, like protein cluster analysis, to classify these viruses based on partial genomic data.

14. What is the role of viral replication strategies in classification?

Viral replication strategies are fundamental to classification, particularly in systems like the Baltimore classification. The way a virus replicates its genome (e.g., DNA to DNA, RNA to DNA, etc.) often reflects deep evolutionary relationships and can be used to group viruses at higher taxonomic levels.

15. How does the concept of "viral species" differ from species concepts in other organisms?

The viral species concept is more fluid than in other organisms. It's based on a combination of genetic similarity, biological properties, and ecological niche, rather than reproductive isolation. This flexibility is necessary due to the rapid evolution and genetic plasticity of viruses, making species boundaries often less distinct.

16. Why are viruses not included in the traditional five-kingdom or three-domain classification systems?

Viruses are excluded from these systems because they are not considered living organisms. They lack cellular structure, cannot reproduce independently, and do not carry out metabolic processes, which are key characteristics of living things in these classification systems.

17. How does virus classification differ from the classification of other organisms?

Virus classification is unique because viruses are not considered living organisms. Unlike other biological entities, viruses are classified based on their genetic material, structure, and mode of replication, rather than evolutionary relationships or cellular characteristics.

18. How has the advent of genomic sequencing technology impacted virus classification?

Genomic sequencing has revolutionized virus classification by providing detailed genetic information. It allows for more accurate identification of viral strains, reveals evolutionary relationships between viruses, and helps in discovering new viruses. This technology has led to frequent updates and refinements in viral taxonomy.

19. How do scientists classify newly discovered viruses?

When classifying new viruses, scientists analyze their genetic material, structure, and host interactions. They compare these characteristics to known viruses and use molecular techniques like PCR and sequencing. The new virus is then placed into existing taxonomic groups or, if unique enough, may form a new group, subject to ICTV approval.

20. How does the concept of "quasispecies" challenge traditional virus classification?

The quasispecies concept describes viruses as a cloud of diverse but related variants rather than a uniform population. This genetic diversity within a single "species" challenges traditional classification methods and highlights the dynamic nature of viral evolution, complicating efforts to define clear taxonomic boundaries.

21. What is virus nomenclature and why is it important?

Virus nomenclature is the system of naming viruses. It's important because it provides a standardized way to identify and classify viruses, enabling clear communication among scientists and facilitating research and disease control efforts.

22. Why do some viruses have names that don't seem to follow standard nomenclature rules?

Some viruses have historical or common names that predate standardized nomenclature. These names often reflect the disease they cause, their discovery location, or other characteristics. While efforts are made to standardize names, some older names persist due to their widespread use in literature and public health communications.

23. What is the role of the International Committee on Taxonomy of Viruses (ICTV)?

The ICTV is responsible for developing, refining, and maintaining a universal virus taxonomy. They establish standardized naming conventions and classification criteria for viruses, ensuring consistency in virus identification and classification worldwide.

24. How do scientists classify viroids and why are they different from viruses?

Viroids are classified separately from viruses because they consist only of a small circular RNA molecule without a protein coat. They are categorized based on their sequence, structure, and biological properties. Unlike viruses, viroids don't encode any proteins and rely entirely on host cellular machinery for replication.

25. What role does host range play in virus classification?

Host range is an important factor in virus classification, especially at lower taxonomic levels. Viruses that infect similar hosts often share genetic and structural similarities. However, host range alone is not sufficient for classification, as some viruses can infect multiple host species or jump between different host types.

26. What are satellite viruses and how are they classified?

Satellite viruses are viruses that require the presence of another virus (the helper virus) to replicate. They are classified based on their genetic material and helper virus dependency. Their classification is unique as they form a separate category that doesn't fit neatly into traditional virus taxonomy, highlighting the diversity of viral entities.

27. How do scientists classify viruses that can infect multiple kingdoms of life?

Viruses that infect multiple kingdoms, like some bacteriophages that can also infect eukaryotic cells, pose a challenge to classification. Scientists consider their genomic structure, replication strategy, and evolutionary history. Often, these viruses are classified based on their primary host or genetic similarities to other viruses, with notes on their broad host range.

28. What is the role of viral proteins in classification?

Viral proteins play a significant role in classification, especially structural proteins that form the capsid or envelope. The arrangement and types of these proteins can be characteristic of certain viral families. Additionally, key enzymes like polymerases are often conserved within viral groups and can be used as markers for classification.

29. What is the significance of viral genome organization in classification?

Genome organization is a key factor in virus classification. It includes aspects like genome type (DNA/RNA, single/double-stranded), segmentation, and the presence of unique genomic features. Similar genome organizations often indicate evolutionary relationships and can help in grouping viruses into higher taxonomic ranks.

30. What is the importance of viral tropism in classification?

Viral tropism, which refers to the specific tissues or cells a virus infects, is an important consideration in classification, especially at lower taxonomic levels. It often correlates with the presence of specific receptor-binding proteins and can provide insights into the virus's evolutionary history and potential for cross-species transmission.

31. What is the role of viral host factors in classification?

While not a primary classification criterion, viral host factors (cellular proteins required for viral replication) can provide insights into virus-host interactions and evolution. Similarities in host factor requirements can indicate relatedness between viruses. This information is often included in detailed virus descriptions and can influence lower-level taxonomic groupings.

32. What is the role of viral vector potential in classification?

The potential of a virus to be used as a vector (for gene therapy or vaccine development) is not a classification criterion but is an important characteristic noted in virus descriptions. Viruses with similar vector potential may share genetic or structural features that make them amenable to modification. This

33. What is the significance of "type species" in virus taxonomy?

A type species serves as the representative or reference for its genus. It typically exhibits the characteristic features of the genus and is often the first or best-studied member. Type species are crucial for comparing and classifying new viruses and for maintaining consistency in viral taxonomy.

34. How does the classification of bacteriophages differ from that of viruses infecting eukaryotes?

Bacteriophage classification has traditionally been based on morphology and genomic structure, with less emphasis on host range compared to eukaryotic viruses. Recent advances in genomics have led to efforts to integrate phage classification with the broader virus taxonomy, but challenges remain due to the vast diversity and rapid evolution of phages.

35. How do scientists classify viruses that undergo frequent genetic recombination?

Classifying viruses that frequently recombine, like influenza viruses, is challenging. Scientists focus on conserved genomic regions and use phylogenetic analysis of multiple genes. They also consider the virus's overall biological properties and may create classification systems that account for the reassortment of genomic segments.

36. How do scientists approach the classification of viruses with segmented genomes?

Classification of viruses with segmented genomes, like influenza viruses, considers the number and nature of segments, as well as the proteins encoded by each segment. Scientists often use a combination of genomic analysis of conserved segments and phylogenetic studies to classify these viruses, taking into account their propensity for genetic reassortment.

37. What is the significance of viral envelope proteins in classification?

Envelope proteins are crucial for virus classification, especially for enveloped viruses. These proteins often determine host range and cell tropism. Their structure and arrangement can be characteristic of certain viral families. Additionally, envelope proteins are important targets for immune responses, making them relevant for both classification and vaccine development.

38. How has the discovery of endogenous viral elements in host genomes affected virus classification?

The discovery of endogenous viral elements (EVEs) in host genomes has provided new insights into viral evolution and classification. EVEs represent "fossil records" of ancient viral infections and can help in understanding the long-term evolution of viruses. They challenge classification by blurring the lines between virus and host genomes and provide evidence for ancient viral lineages.

39. What is the role of viral pathogenicity in classification?

While pathogenicity is not a primary criterion for classification, it can be an important consideration, especially for viruses of medical or agricultural importance. Viruses causing similar diseases or showing similar pathogenic mechanisms may be grouped together. However, classification systems primarily focus on genetic and structural features rather than pathogenic effects.

40. How do scientists classify viruses that can exist in both lytic and lysogenic cycles?

Viruses capable of both lytic and lysogenic cycles, like many bacteriophages, are classified based on their genetic material and overall structure rather than their life cycle. However, the ability to switch between these cycles is noted in their biological description and can be important for understanding their ecological role and evolution.

41. What is the significance of viral genetic recombination in classification challenges?

Genetic recombination in viruses can create new strains that challenge existing classification schemes. It can lead to viruses with mosaic genomes, combining features of different viral groups. Scientists address this by focusing on conserved genomic regions and considering the evolutionary history of different parts of the viral genome separately.

42. How does the classification of RNA viruses differ from that of DNA viruses?

RNA viruses are often classified differently from DNA viruses due to their higher mutation rates and greater genetic diversity. Classification of RNA viruses places more emphasis on replication strategies and genomic organization. The Baltimore classification system is particularly useful for RNA viruses, grouping them based on their mRNA production methods.

43. What role do viral enzymes play in classification?

Viral enzymes, especially those involved in replication like polymerases, are important for classification. These enzymes are often highly conserved within viral groups and can provide insights into evolutionary relationships. The presence or absence of certain enzymes (e.g., reverse transcriptase in retroviruses) can be defining features for some viral classes.

44. How has the discovery of virophages impacted virus classification?

Virophages, which are viruses that infect other viruses, have challenged traditional classification systems. They represent a unique category that doesn't fit neatly into existing taxonomic frameworks. Their discovery has led to discussions about expanding virus classification to include new categories and rethinking the nature of viral parasitism.

45. What is the importance of viral capsid structure in classification?

Capsid structure is a fundamental criterion in virus classification. The shape (icosahedral, helical, etc.) and composition of the capsid can be characteristic of certain viral families. Capsid proteins are often conserved within viral groups and can provide insights into evolutionary relationships, making them valuable for both structural and genetic classification approaches.

46. How do scientists approach the classification of viruses that undergo antigenic drift and shift?

For viruses undergoing antigenic drift and shift, like influenza viruses, classification involves monitoring genetic changes over time. Scientists use phylogenetic analysis of key genes (e.g., hemagglutinin and neuraminidase for influenza) to track evolutionary changes. Classification systems for these viruses often include information on serotypes or genetic clades to reflect their dynamic nature.

47. How does the concept of "viral quasispecies" impact classification efforts?

The concept of viral quasispecies, which describes viruses as a cloud of related variants, challenges traditional species definitions. It emphasizes the dynamic nature of viral populations and their rapid evolution. Classification systems must account for this genetic diversity within a single "species," often leading to more flexible definitions of viral taxa.

48. What is the significance of viral genome size in classification?

Genome size can be an important factor in virus classification, especially at higher taxonomic levels. It can indicate the complexity of the virus and its replication strategy. For example, the discovery of giant viruses with large genomes has led to new classification categories. However, genome size alone is not sufficient for classification and is considered alongside other genetic and structural features.

49. How do scientists classify viruses that exhibit complex life cycles involving multiple hosts?

Viruses with complex life cycles involving multiple hosts, like some arboviruses, are classified based on their genetic material and structural features rather than their host range. However, their unique life cycles are noted in their biological descriptions. Classification may consider the primary host or the host in which the virus replicates most efficiently.

50. What is the role of viral entry mechanisms in classification?

Viral entry mechanisms, while not primary classification criteria, can provide insights into viral evolution and host range. Similarities in entry mechanisms can indicate relatedness between viruses. This information is often included in detailed virus descriptions and can influence lower-level taxonomic groupings, especially when correlated with genetic similarities.

51. How has the discovery of CRISPR systems in prokaryotes influenced our understanding of virus evolution and classification?

The discovery of CRISPR systems, which act as a prokaryotic immune system against viruses, has provided new insights into virus-host coevolution. It has revealed a long history of interactions between viruses and their hosts, influencing our understanding of viral diversity and evolution. While not directly used for classification, CRISPR studies contribute to our knowledge of viral evolutionary history and host range.

52. What is the significance of viral latency in classification?

Viral latency, the ability of a virus to remain dormant in host cells, is not a primary classification criterion but is an important biological feature noted in virus descriptions. Viruses capable of latency, like herpesviruses, may share certain genetic elements that enable this ability. This characteristic can influence lower-level taxonomic groupings and is important for understanding viral life cycles and host interactions.

53. How do scientists approach the classification of defective viral particles?

Defective viral particles, which lack some components necessary for independent replication, are not classified separately from their parent viruses. They are considered part of the viral population diversity. However, their existence and properties are noted in detailed virus descriptions, as they can play important roles in viral evolution and pathogenesis.