Classification of Viruses: Study Notes

For the complete syllabus, check out our PMDC MDCAT Syllabus 2025 article.

Viruses are obligate parasites i.e. they can only replicate in their host. They are classified on three basis:

1. Host
2. Shape/Capsid
3. Genome

Classification based on host:

Animal viruses:​

- Viruses that infect animal hosts

- Examples: Rabies, Polio, Mumps, Chickenpox, Smallpox, and Influenza

Plant viruses:

- Viruses that exhibit living characteristics when attached to plants

- Examples: Tobacco mosaic virus (TMV), Banana streak virus, Carrot thin leaf virus

Bacterial viruses:

- Known as bacteriophages

- Examples: T1, T2, T3, and T4

Classification based on Shape/Capsid

Helical viruses, like the Tobacco Mosaic Virus, which infects a number of different types of plants, have a slinky-shaped capsid that twists around and encloses its genetic material.

Polyhedral viruses, like adenoviruses, which are known to cause a range of illnesses from pink eye to pneumonia, are composed of genetic material surrounded by a many-sided capsid, usually with 20 triangular faces.

Spherical viruses, like the infamous Coronavirus, are essentially helical viruses enclosed in a membrane known as an envelope, which is spiked with sugary proteins that assist in sticking to and entering host cells.

Complex viruses, like bacteriophages, which infect and kill bacteria, resemble a lunar lander, and are composed of a polyhedral “head” and a helical body (or “tail sheath”), and legs (or “tail fibers”) that attach to a cell membrane so that it can transfer its genetic material.

Classification based on genome

In 1971, David Baltimore, a renowned virologist who received the Nobel Prize, classified viruses into seven groups based on their genetic makeup. Recently, in 2018-2019, the Baltimore classification was slightly modified to account for evolutionary aspects, revealing that certain virus groups share common ancestors. The modified classification of viruses is as follows:

DNA viruses

These viruses possess Deoxyribonucleic acid (DNA) as their genetic material and are further divided into two groups:

1. Double-stranded DNA viruses: Their DNA consists of two strands. Inside the host cell nucleus, they utilize enzymes from the host to produce messenger RNA (mRNA). While some of these viruses can cause cancer, none of them are known to infect plants. For example, Herpes is a double-stranded DNA virus.
2. Single-stranded DNA viruses: These viruses contain a single strand of DNA. In the host cell, they first become double-stranded and then use transcription to generate mRNA. The new virus progeny subsequently revert to having a single-stranded DNA structure. Parvoviruses are examples of single-stranded DNA viruses.

RNA viruses

These viruses have RNA as their genetic material and are grouped as follows:

1. Double-stranded RNA viruses: These viruses possess double-stranded RNA as their genome. Inside the host cell, they use cellular enzymes to create single-stranded mRNA. This mRNA is then utilized for translation or replication of the double-stranded RNA, which serves as the genetic material for producing new virus progeny. Reoviruses are an example of this group.
2. Positive sense single-stranded RNA viruses: These viruses contain a single-stranded RNA. The term "positive sense" indicates that their RNA functions as messenger RNA (mRNA) and can be directly translated by the host cell without the need for transcription. Examples of positive-sense single-stranded RNA viruses include Coronavirus, Dengue virus,Hepatitis C virus, Togavirus and Picornavirus
3. Negative sense single-stranded RNA viruses: These viruses also have single-stranded RNA. Upon entering the host cell, they generate mRNA from their RNA using cellular machinery for translation purposes. Rhabdo virus and Paramyxovirus are examples of negative-sense single-stranded RNA viruses.

Reverse Transcribing Viruses

The process of converting RNA into DNA is called reverse transcription. This group of viruses can be further classified as follows:

1. Single-stranded RNA viruses with a DNA intermediate: These viruses have single-stranded RNA with a positive sense, but they need to replicate using a DNA intermediate. Inside the host cell, an enzyme called reverse transcriptase helps the RNA to form DNA. This DNA is then integrated into the host's genetic material for transcription and translation, with the assistance of an enzyme called integrase. Retroviruses like HIV are examples of this group, which have two single-stranded RNA molecules.
2. Double-stranded DNA viruses with an RNA intermediate: These viruses have a DNA genome, but during their replication cycle, RNA is formed. This RNA is then used for reverse transcription to replicate the viral genome inside the protective protein coat. Hepatitis B is an example of a virus in this group.

In addition to the classification based on their genome, viruses can also be grouped according to their relationship with their host. For example, bacteriophages infect bacteria, phytophages infect plants (such as TMV), and zoophages infect animals and humans (such as HIV and COVID-19).

Explore our MDCAT webapp for tests, video lectures, and complete prep resources.