Genome Architecture as the Basis of Systematics
The fundamental criterion for virus classification is the detailed characterization of their genetic material—the nucleic acid (NA). First, the type of molecule is evaluated: the genome may consist of either DNA or RNA. Next, the structural diversity of the nucleic acids is considered across several key parameters. The number of strands is of immense importance, distinguishing between single-stranded and double-stranded viral genomes.
For single-stranded molecules, polarity is a critically important characteristic. There are agents with positive polarity (+), negative polarity (–), as well as mixed-polarity variants known as ambisense viruses (+/–). Topology (the spatial form of the molecule) also plays a significant role: it can be elongated linear or closed circular (ring-shaped). In addition, the integrity of the genetic material is assessed, which can be either intact (non-fragmented) or divided into separate segments.
Additional Taxonomic Markers
In addition to nucleic acid properties, a wide range of secondary characteristics is used for classification. Morphological features include the overall virion size, the type of symmetry of its structure, and the exact number of capsomeres forming the capsid. Special attention is paid to the presence or absence of an outer lipid envelope—the viral envelope (peplos/supercapsid).
Researchers also rely on the physicochemical properties of viral particles. For example, an important differential feature is the sensitivity of the virion to deoxycholate and ether. Equally significant are the biological properties of the infectious agent: the specifics of its antigenic structure and its preferred site of replication within the infected cell.
Taxonomy, Nomenclature, and the Baltimore System
In modern scientific classification, viruses are grouped based on the mechanisms of messenger RNA (mRNA) synthesis. This concept is closely linked to the classification proposed by Nobel laureate David Baltimore. The core principle of the Baltimore system is the division of viruses into groups according to their mRNA synthesis pathways, which largely dictates the logic of intracellular parasitism.
An interesting feature of viral taxonomy is the complete absence of binomial nomenclature universally applied to bacteria (double Latin names are not used). Instead, a strict system of suffixes is employed for taxonomic ranks:
- Family membership is designated by the suffix -viridae.
- Subfamilies are assigned the suffix -virinae.
- Genus names end in -virus.
Classification of Major Pathogens and Their Clinical Role
Viruses infect all forms of life, including plants and bacteria (bacteriophages). For humans, they are major causative agents of infectious pathologies and also play an active role in carcinogenesis. Many of them are capable of causing post-infectious complications such as myocarditis, pancreatitis, and various immunodeficiency states.
Several viruses possess the ability for vertical (transplacental) transmission, causing fetal injury. Prominent examples include cytomegalovirus and rubella virus (belonging to the family Togaviridae).
Of immense clinical significance is the group of negative-sense single-stranded RNA viruses, which includes causative agents of severe infections:
- Bornaviridae: Borna disease virus.
- Filoviridae: Ebola and Marburg viruses (causative agents of African hemorrhagic fevers).
- Paramyxoviridae: mumps virus, parainfluenza viruses, respiratory syncytial virus, and measles virus (can lead to subacute sclerosing panencephalitis).
- Rhabdoviridae: rabies virus.
- Orthomyxoviridae: influenza viruses A, B, and C.
- Bunyaviridae: causative agents of Crimean-Congo hemorrhagic fever and hemorrhagic fever with renal syndrome (HFRS).
- Arenaviridae: lymphocytic choriomeningitis virus.
- Deltavirus: defective hepatitis D virus, which requires coinfection with hepatitis B virus.
A special category consists of viruses utilizing reverse transcription. These include RNA-containing retroviruses (family Retroviridae), which encompass HIV (the causative agent of AIDS), as well as DNA viruses of the family Hepadnaviridae, a typical representative of which is the hepatitis B virus.