A virus is a small infectious agent that can replicate only inside a living cell. Viruses contain genetic material, either DNA or RNA, packaged in a protective protein coat. Some also have a lipid envelope taken from a host-cell membrane.
Viruses infect animals, plants, fungi, bacteria, archaea, and other organisms. They are central to infectious disease, ecology, evolution, molecular biology, vaccine development, and biotechnology.
Structure
A complete virus particle outside a cell is called a virion. The basic parts are:
- A genome made of DNA or RNA.
- A protein coat called a capsid.
- In some viruses, an envelope containing lipids and viral proteins.
- Surface proteins that help attach to host cells.
The genome may be single-stranded or double-stranded, segmented or unsegmented, circular or linear. This affects how the virus replicates and how it is classified.
Replication
Viruses do not have the full machinery needed for independent metabolism or reproduction. They must enter a host cell and use that cell's machinery to make viral components.
A simplified replication cycle includes:
- Attachment to a suitable host-cell receptor.
- Entry into the cell or delivery of the genome.
- Release of the viral genome inside the cell.
- Genome replication and viral protein production.
- Assembly of new virions.
- Release by cell lysis, budding, exocytosis, or another route.
Different viruses use different strategies. For example, retroviruses use reverse transcriptase, many DNA viruses replicate in the nucleus, and many RNA viruses replicate in the cytoplasm.
Classification
Viruses are classified by genome type, structure, replication strategy, host range, evolutionary relationships, and taxonomic evidence. The International Committee on Taxonomy of Viruses provides official virus taxonomy resources, including current species lists and taxonomy releases.
Common broad categories include:
- DNA viruses.
- RNA viruses.
- Reverse-transcribing viruses.
- Enveloped and non-enveloped viruses.
- Animal, plant, fungal, bacterial, and archaeal viruses.
Virus names and virus species names are not always the same thing. ICTV guidance treats taxonomy as a formal scientific system, while common virus names are often used in medicine and public communication.
Viral Disease
Many viruses cause disease, but many others do not cause obvious disease in a particular host. Disease depends on the virus, the host, immune status, exposure route, dose, and tissue affected.
Human viral diseases include influenza, measles, COVID-19, HIV infection, viral hepatitis, chickenpox, shingles, rabies, dengue, norovirus infection, and many common respiratory infections.
Viral disease can be caused by:
- Direct cell damage.
- Immune-mediated injury.
- Persistent infection.
- Cancer-promoting changes in infected cells.
- Secondary complications, such as bacterial infection after a viral illness.
Prevention
Prevention depends on the virus. Measures can include vaccination, ventilation, hand hygiene, safer sex, blood screening, vector control, isolation during infectious periods, protective equipment, and animal vaccination.
Vaccination is one of the most important tools against viral disease. Some vaccines prevent infection very effectively. Others mainly reduce severe disease, hospitalisation, or death.
Treatment
Antibiotics do not treat viral infections. They act against bacteria, not viruses. Antiviral medicines are available for some viral infections, including HIV, influenza, herpesviruses, hepatitis B, hepatitis C, and COVID-19, but many viral infections are managed with supportive care.
Antiviral treatment works best when it targets a specific step in the viral life cycle, such as entry, genome replication, protein processing, or release. Resistance can develop, especially when viruses mutate rapidly or treatment pressure is incomplete.
Role in Science
Viruses have shaped modern biology. They have helped researchers understand genes, transcription, translation, cancer biology, immunity, and cell signalling. Viral tools are used in gene therapy, vaccines, protein production, and laboratory delivery systems.
Viruses also affect evolution. They can move genetic material between organisms, apply selection pressure to host populations, and leave viral sequences in host genomes over long periods.
See Also
References
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