VIRUS

WHAT IS A VIRUS?

A virus is the smallest type of parasite to exist and is typically within the size range of 0.02 to 0.3 micrometers (μm) in size; however, some viruses can be as large as 1 μm.

The contents of a virus

Viruses consist of short sequences of nucleic acid, which can be in the form of ribonucleic acid (RNA) or deoxyribonucleic acid (DNA) as their genetic material. As compared to most other organisms where DNA is always a double-stranded structure, viruses are unique because their DNA or RNA material can be either single-stranded or double-stranded.

The typical structure of a virion, which is the term used to describe a single virus, includes an outer shell that is otherwise referred to as the protein capsid or membrane. The primary function of this outer shell is to protect the virion’s genetic information from physical, chemical, or enzymatic damage.

Classifying human viruses

A virus is often classified according to its physicochemical properties, genome structure, size, morphology, and molecular processes. In terms of their genetic material, viruses are classified according to whether they are RNA or DNA viruses and the strandedness of their genetic material, which can include double-stranded (ds), single-stranded (ss), or partially ds. Furthermore, ss viruses will also be classified as to whether they are positive ss, negative ss, or negative with ambisense viruses.

To date, five dsDNA human virus families have been identified, of which include adenoviridae, herpesviridae, papillomaviridae, parvoviridae, and poxivirdae. Picobirnaviridae, picornaviridae, and reoviridae are the three human dsRNA viruses that have been identified.

As compared to DNA viruses, there have been many more RNA viruses that have infected humans. These include nine negative ssRNA and eight positive ssRNA virus families.

Capsid morphology including icosahedral, helical, or complex shapes can also be used to classify a given virus. The presence or absence of an envelope on the virus is also incorporated into its classification