RNA virus

Genome types and classification

RNA viruses show several genome organizations: single‑stranded positive‑sense RNA (+ssRNA), single‑stranded negative‑sense RNA (−ssRNA), double‑stranded RNA (dsRNA), and single‑stranded RNA that is reverse transcribed to DNA during replication (ssRNA‑RT, the retroviruses). These groups form key categories in the Baltimore classification introduced by David Baltimore, a Nobel Prize laureate. Representative Baltimore groups relevant to RNA genomes include:

• Class III: double‑stranded RNA (dsRNA) viruses
• Class IV: positive‑sense single‑stranded RNA (+ssRNA) viruses, whose genome can function as messenger RNA
• Class V: negative‑sense single‑stranded RNA (−ssRNA) viruses, which must synthesize complementary mRNA before translation
• Class VI: single‑stranded RNA viruses with a DNA intermediate (ssRNA‑RT), the retroviruses such as retroviruses

Replication and molecular biology

Most RNA viruses encode an RNA‑dependent RNA polymerase (RdRp) because host cells typically lack the enzymes required to copy RNA genomes. Retroviruses instead carry reverse transcriptase, an RNA‑dependent DNA polymerase that copies viral RNA into DNA. Individual nucleotides of the viral genome are assembled by these polymerases during replication. Compared with many cellular DNA polymerases, many viral RdRps have limited proofreading, which results in higher mutation rates and rapid viral evolution. Some groups, notably certain large nidoviruses such as coronaviruses, possess accessory proteins that improve replication fidelity and permit larger genomes.

Structure, segmentation and genetic exchange

Virions of RNA viruses vary in structure from simple, non‑enveloped icosahedral capsids to enveloped particles with surface glycoproteins. Genomes can be monopartite (single RNA molecule) or segmented into separate RNA segments; segmented genomes allow reassortment, a form of genetic exchange that can occur when two related viruses co‑infect the same cell and swap segments. Reassortment and recombination are important mechanisms that generate new genetic variants and can change host range or antigenic properties.

Evolution, mutation and epidemiology

The comparatively high mutation rates of many RNA viruses facilitate rapid adaptation to new hosts, immune responses and antiviral drugs. This adaptability contributes to antigenic drift in influenza and other rapidly changing viruses. Larger reorganization events such as reassortment or recombination can produce abrupt changes (sometimes called antigenic shift), with potential public‑health consequences if a novel variant spreads widely. RNA viruses are commonly implicated in zoonotic spillover, where a virus moves from an animal reservoir into humans.

Disease burden and control

RNA viruses are responsible for a broad range of acute and chronic diseases in humans, livestock and crops. Control measures include surveillance, vaccination, antiviral drugs, infection control practices and public health interventions. Vaccine development must account for viral diversity and evolution; for some RNA viruses, repeated updates to vaccine strains or alternative strategies are required to maintain protection.

Research applications and laboratory use

RNA viruses and their components are important tools in molecular biology, biotechnology and gene therapy research. Retroviral and lentiviral vectors are commonly used to deliver genetic material into cells for experimental and therapeutic purposes. Engineered RNA viruses serve as platforms for vaccine development and as vectors for oncolytic therapies in experimental settings.

Taxonomy and naming

In addition to the Baltimore scheme, formal viral taxonomy is maintained by the International Committee on Taxonomy of Viruses (ICTV), which organizes viruses into ranked categories such as orders and families. These standardized names and groupings support research, diagnostics and public health reporting. For current, detailed classifications and species lists, consult specialist reviews and curated databases and resources that cover RNA virus diversity and nomenclature (general RNA virus resources).