Baltimore classification in the context of "Reverse transcription"

Virus classification is the process of naming viruses and placing them into a taxonomic system similar to the classification systems used for cellular organisms.

Viruses are classified by phenotypic characteristics, such as morphology, nucleic acid type, mode of replication, host organisms, and the type of disease they cause. The formal taxonomic classification of viruses is the responsibility of the International Committee on Taxonomy of Viruses (ICTV) system, although the Baltimore classification system can be used to place viruses into one of seven groups based on their manner of mRNA synthesis. Specific naming conventions and further classification guidelines are set out by the ICTV.

Positive-strand RNA viruses (+ssRNA viruses) are a group of related viruses that have positive-sense, single-stranded genomes made of ribonucleic acid. The positive-sense genome can act as messenger RNA (mRNA) and can be directly translated into viral proteins by the host cell's ribosomes. Positive-strand RNA viruses encode an RNA-dependent RNA polymerase (RdRp) which is used during replication of the genome to synthesize a negative-sense antigenome that is then used as a template to create a new positive-sense viral genome.

Positive-strand RNA viruses are divided between the phyla Kitrinoviricota, Lenarviricota, and Pisuviricota (specifically classes Pisoniviricetes and Stelpavirictes) all of which are in the kingdom Orthornavirae and realm Riboviria. They are monophyletic and descended from a common RNA virus ancestor. In the Baltimore classification system, +ssRNA viruses belong to Group IV.

The bat virome is the group of viruses associated with bats. Bats host a diverse array of viruses, including all seven types described by the Baltimore classification system: (I) double-stranded DNA viruses; (II) single-stranded DNA viruses; (III) double-stranded RNA viruses; (IV) positive-sense RNA viruses; (V) negative-sense RNA viruses; (VI) positive-sense RNA viruses that replicate through a DNA intermediate; and (VII) double-stranded DNA viruses that replicate through a single-stranded RNA intermediate. The greatest share of bat-associated viruses identified as of 2020 are of type IV, in the family Coronaviridae.

Bats harbor many viruses and a few of them are zoonotic, or capable of infecting humans and causing disease, thus, are considered important emerging viruses. These zoonotic viruses include the rabies virus, SARS-CoV, MERS-CoV, Marburg virus, Nipah virus, and Hendra virus. While research strongly indicates that SARS-CoV-2 originated in bats, it is unknown how it was transmitted to humans, or if an intermediate host was involved. It has been speculated that bats may have a role in the ecology of Ebola virus, though this is unconfirmed. A closely-related ebolavirus, Bombali ebolavirus, has been detected in insectivorous bats. While transmission of rabies from bats to humans usually occurs from bites or scratches, most other zoonotic bat viruses are transmitted by direct contact with infected bat fluids like urine, guano, or saliva, or through contact with an infected, non-bat intermediate host. There is no firm evidence that butchering or consuming bat meat can lead to viral transmission, though this has been speculated because such activity causes aerosols that may facilitate respiratory transmission.