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Virus: Definition And Types

Viruses are microscopic parasites that live inside the cells of living beings.
A virus is an infectious agent that affects living cells of other organisms


A virus is a parasitic intracellular entity that causes thousands of diseases every day, many of them fatal to humans.

In this article, we will see a definition, which are the 10 most common types and how to classify them.

What is a virus?

A virus is an intracellular parasite that needs to infect a living cell in order to reproduce. This process is called infection.

Although they share several characteristics with living organisms, such as the presence of genetic material (DNA or RNA), they are not considered living beings, yet the latter statement is in constant debate.

They are composed only of an outer layer (capsid), the genome and, in some cases, some enzymes. Together, these make up the virion or particle.

Zika, human papilloma, rabies, AIDS or influenza are caused by them. They also infect animals, plants, bacteria, and fungi. Those that cause diseases are the RNA ones and are the most dangerous because they mutate more easily and have greater resistance.

Viruses are resistant to antibiotics, for they act on a bacterial cell wall.

Virus vs bacteria

Bacteria can also cause infectious diseases such as salmonellosis. The main difference is that bacteria are living organisms that are metabolically active, sensitive to antibiotics and reproduce by cell division.

They are noticeably smaller than bacteria and their structure is different.


Scientists have classified them according to the type of genetic material they contain. Broad categories include single and double-stranded DNA and single and double-stranded RNA.

The following list shows the most common types divided according to the disease they cause.

1. Papovavirus

This type includes two groups: papilloma and polyomaviruses. The first ones are the smallest double-stranded DNA ones, causing several types of benign (non-cancerous) warts, including plantar warts (those that form on the soles of the feet) and genital warts.

Also, they influence the emerge of cervical cancer, that accounts for 7% of all female cancers.

2. Retrovirus

They belong to the double-stranded RNA type and contain an enzyme called reverse transcriptase. Inside the host cell, the virus uses reverse transcriptase to make a DNA copy from its RNA genome.

In all other organisms, RNA is synthesized from DNA. HIV is the best-known one, being responsible for AIDS.

3. Variola virus

This is the most complex type known. They have large amounts of genetic material and fibrils anchored to the outside of the viral capsid that help bind to the host cell. They contain double-stranded DNA.

Smallpox is the only disease eradicated thanks to human action throughout history.

4. Herpesviridae

These are double-stranded, wrapped DNA types. Of the more than 50 ones that exist, only eight cause disease in humans.

These include herpesviruses 1 and 2, which cause cold sores and genital herpes, and type 3 or the varicella-zoster one (VZV), which causes chickenpox and shingles (herpes zoster).

5. Adenovirus

They attack the respiratory, intestinal and ocular cells of animals. There are more than 40 types of human adenoviruses, and they contain double-stranded DNA.

The ones that affect respiratory cells cause bronchitis, pneumonia, and tonsillitis. Also, some cases of appendicitis are also caused by this subtype.

As far as eye diseases are concerned, they include conjunctivitis, as well as pharyngoconjunctival fever, which can be contracted in pools with low chlorination.

6. Hepadnaviridae

These types cause several diseases, including hepatitis B. Hepatitis B is a chronic, debilitating disease of the liver and immune system. Many people who get the disease become carriers of the virus, able to transmit it through body fluids such as blood, semen, and vaginal liquids.

7. Orthomyxoviridae

They are the cause of the highly contagious flu. The most common complication of influenza is pneumonia.

These types are single-stranded RNA spherical ones. Three strains of the influenza virus (A, B and C) cause disease in humans. However, there is a vaccine against it.

This vaccination should be carried out during the fall months, as the relevant antibodies reach optimal numbers in the winter.

This type of virus is very common, and during flu season it is recommended to get the influenza vaccine


8. Rhinovirus

They cause the common cold and belong to the same family as enteroviruses. There are 113 types and each one differs slightly in the composition of its capsid.

9. Filoviridae

Ebola and Marburg (the most common ones) are the two most lethal human viruses. Both are transmitted through contact with body fluids.

10. Rhabdoviridae

They are single-stranded, bullet-shaped RNA viruses, being responsible for rabies, a disease that is transmitted to humans by the bite of an infected animal.


It considers the characteristics of the genome, the virion form, and macromolecular composition and other properties such as antigenicity and host range.

A classification scheme based on the type of nucleic acid (DNA or RNA) present in the virus particle and the method of genome replication was devised by David Baltimore, co-discoverer of reverse transcriptase.



Garry, R. F. (2003). Virus. In R. Robinson (Ed.), Genetics (Vol. 4, pp. 164-171). New York: Macmillan Reference USA.

Scogna, K. (2004). Virus. In K. L. Lerner & B. W. Lerner (Eds.), The Gale Encyclopedia of Science (3rd ed., Vol. 6, pp. 4229-4236). Detroit: Gale.