A virus is an infectious biological agent that reproduces only within a living cell. Viruses carry genetic information and direct the production of new viral components, but depend on cellular machinery and resources for replication. They lack independent metabolism and their own protein-synthesizing apparatus. Their hosts include animals, plants, fungi, bacteria, and archaea. A complete virus particle, called a virion, transfers the viral genome between susceptible cells or hosts. Viruses are therefore distinct from cellular microorganisms, even though both may cause infection. (ncbi.nlm.nih.gov)
Structure and genomes
The basic virion consists of a genome enclosed in a protein shell called a capsid. Some viruses also possess a lipid envelope derived from a host-cell membrane and containing viral surface proteins. Capsids protect genetic material and, in many viruses, participate in attachment to cells. Their arrangements include helical and icosahedral structures; other viruses have more complex architectures. An envelope is not equivalent to the cell membrane of an independently functioning cell. (ncbi.nlm.nih.gov)
A viral genome consists of DNA or RNA, which may be single-stranded or double-stranded, linear or circular, and contained in one molecule or divided into segments. Genomes encode structural proteins and other products involved in replication or interactions with the host. Some virions also carry enzymes needed during the early stages of infection. This diversity of genome organization underlies major differences in viral replication strategies. (ncbi.nlm.nih.gov)
Replication and persistence
Replication generally involves attachment, entry, exposure of the genome, synthesis of viral components, assembly, and release. Attachment depends on interactions between viral structures and cellular receptors. Receptor availability helps determine which cells a virus can enter, although successful replication also requires a suitable intracellular environment. Depending on the virus, entry may involve membrane fusion, uptake into vesicles, or delivery of genetic material across the cell boundary. (ncbi.nlm.nih.gov)
Once the genome becomes accessible, viral gene expression produces the molecules needed for replication. Host ribosomes translate viral messenger RNA into proteins. The machinery that copies viral genomes varies: some viruses rely extensively on cellular enzymes, while others encode their own polymerases. Newly synthesized genomes and proteins assemble into particles, which may leave through cell rupture or budding. Viral reproduction is thus an assembly process rather than cell division. (ncbi.nlm.nih.gov)
Infection need not immediately produce new particles or destroy the cell. During viral latency, a viral genome persists with restricted activity and can later reactivate. Herpesviruses provide examples of this pattern. Latency differs from persistent productive infection, in which viral replication continues over an extended period. (ncbi.nlm.nih.gov)
Classification
The Baltimore classification groups viruses by genome type and the pathway used to generate messenger RNA. Its seven groups are double-stranded DNA, single-stranded DNA, double-stranded RNA, positive-sense single-stranded RNA, negative-sense single-stranded RNA, RNA viruses using reverse transcription, and DNA viruses using reverse transcription. Positive-sense RNA can function as messenger RNA; negative-sense RNA requires production of a complementary strand suitable for translation. (ncbi.nlm.nih.gov)
This functional scheme is distinct from formal viral taxonomy. The International Committee on Taxonomy of Viruses establishes taxa in a hierarchy extending from species to realms. Classification considers genome sequences, evolutionary relationships, and other characteristics. Sequence comparison and phylogenetics are especially important. Categories based simply on host or transmission route may be useful without constituting evolutionary groups: viruses infecting bacteria, collectively called bacteriophages, do not form a single taxonomic lineage. (ictv.global)
Evolution and ecological roles
Viral populations change through mutation, genetic recombination, and, in viruses with segmented genomes, reassortment. Reassortment exchanges complete genome segments when compatible viruses infect the same cell. Mutations can be harmful, neutral, or beneficial in a particular environment; their persistence depends on replication, transmission, and selection. Genetic changes may alter host interactions or the recognition of viral proteins by immune defenses. (ncbi.nlm.nih.gov)
Viruses also influence ecosystems independently of human disease. In aquatic environments, infection and rupture of microbial cells release organic material into surrounding water. This redirects material through microbial processing and can affect nutrient recycling and the carbon cycle. Experimental work shows that the effects differ between environmental conditions, so viral activity does not have one uniform consequence for the fate of organic matter. (pubmed.ncbi.nlm.nih.gov)
Infection, immunity, and detection
Viral infection and disease are not synonymous. Infection may remain clinically inapparent, while disease can arise from direct cellular damage or from host responses to infection. Which tissues are affected depends on viral properties, accessibility, cellular susceptibility, and host defenses. Viruses differ substantially in their routes of entry, spread within the body, and release into the environment. (ncbi.nlm.nih.gov)
The immune system counters infection through several mechanisms. Interferons help induce antiviral activity in cells. Antibodies can recognize viral structures, while T cells contribute cellular defenses. Vaccines expose the immune system to antigens or instructions for producing them, establishing responses and immune memory without requiring the corresponding disease. Antibiotics do not treat viral infections. (ncbi.nlm.nih.gov)
Laboratory diagnosis may involve detecting viral genetic material through polymerase chain reaction, identifying viral antigens, measuring antibodies, or growing a virus in susceptible cells. These methods examine different aspects of infection: genetic or antigen tests detect viral components, antibody tests detect a host response, and culture demonstrates replication under the conditions used. The applicable method depends on the virus and the specimen available. (ncbi.nlm.nih.gov)