Fundamental Differences and Virion Morphology
A mature, fully formed viral particle is called a virion. As "filterable agents," viruses are comparable in size to the thickness of a bacterial cell wall. The smallest agents are polioviruses and parvoviruses (about 18–20 nm), while the largest are poxviruses (e.g., Variola virus), reaching up to 350 nm in length.
Virions vary widely in shape. Several main morphological types are recognized:
- Spherical (characteristic of HIV, polioviruses, herpesviruses).
- Rod-shaped (a classic example is tobacco mosaic virus).
- Bullet-shaped (rounded at one end and flat at the other, as in rabies virus).
- Filamentous (long, convoluted structures typical of filoviruses).
- Spermatozoon-shaped (characteristic structure of many bacteriophages).
Genetic Organization
The viral genome consists of DNA or RNA, which can be linear, circular, or fragmented. In terms of ploidy, the vast majority of viruses are haploid (possessing a single set of genes), with only retroviruses possessing a diploid genome. The amount of genetic information varies from a few genes to hundreds.
Inside an infected cell, the viral genome can exist as a provirus (when the genetic material integrates into the host chromosome) or in a plasmid-like form (autonomous presence of nucleic acid in the cytoplasm, typical of herpesviruses).
The polarity of single-stranded RNA viruses is of particular importance:
- Positive-sense (+RNA): functions simultaneously as hereditary material and messenger RNA. Such a strand is infectious on its own and can directly initiate protein translation on host ribosomes.
- Negative-sense (-RNA): carries exclusively hereditary information. To initiate protein synthesis, a complementary positive strand must first be synthesized on its template.
- Ambisense viruses: the genome contains regions of both polarities.
Principles of Structural Classification
Basic taxonomy relies on three main criteria: the type of nucleic acid (DNA or RNA), genome structure (single- or double-stranded), and the presence of an outer lipoprotein envelope.
- Simple ("naked") viruses consist only of nucleic acid and a protective protein coat—the capsid. The capsid itself is formed from individual subunits called capsomeres. Examples include hepatitis A virus and polioviruses.
- Enveloped viruses have an additional layer over the nucleocapsid—the supercapsid (envelope), which is often equipped with special spikes. Enveloped viruses include coronaviruses, orthomyxoviruses, and herpesviruses.
Key Families of Pathogenic Viruses
Viral taxonomy includes several major clinically significant groups.
DNA Virus Group: Among non-enveloped double-stranded viruses are Adenoviridae (causing acute respiratory infections), Papillomaviridae (warts and oncology), and Polyomaviridae. Non-enveloped single-stranded DNA viruses include the tiny Parvoviridae (agents of erythema infectiosum) and Anelloviridae. Among enveloped DNA viruses, the most clinically relevant are the complex and large Poxviridae, spherical Herpesviridae (herpes simplex viruses, cytomegalovirus, Epstein-Barr virus, Varicella Zoster), and small Hepadnaviridae.
RNA Virus Group: Double-stranded non-enveloped RNA viruses include Reoviridae, specifically rotaviruses (the leading cause of gastroenteritis). Non-enveloped single-stranded viruses are represented by small families such as Picornaviridae (hepatitis A), Caliciviridae (noroviruses), Hepeviridae, and Astroviridae. Enveloped single-stranded RNA viruses form the largest group. This includes spherical forms: Coronaviridae (agents of severe acute respiratory infections), vector-borne Flaviviridae (yellow fever, tick-borne encephalitis, hepatitis C), Orthomyxoviridae, Retroviridae, and Togaviridae. Bullet-shaped forms include Rhabdoviridae, and filamentous forms include Filoviridae.