wirus

Wirus is a submicroscopic infectious agent that cannot replicate outside a living cell. It consists of genetic material—DNA or RNA—enclosed in a protein coat called a capsid; some viruses also have a lipid envelope derived from the host cell membrane. Virus is not considered a living organism by many definitions because it lacks independent metabolism and cannot grow outside a host. They infect organisms across all domains of life, including bacteria (bacteriophages), plants, and animals. Viral genomes can be DNA or RNA, single- or double-stranded, and may be circular or segmented. Replication occurs only inside a host cell, using the host's transcription and translation machinery. The general life cycle involves attachment to a receptor, entry, genome replication, production of viral proteins, assembly, and release by lysis or budding. Transmission routes vary and include aerosols, droplets, bodily fluids, fecal-oral spread, vectors, or contaminated surfaces. The clinical impact ranges from asymptomatic infection to severe disease; notable illnesses include influenza, HIV/AIDS, hepatitis, and diseases caused by coronaviruses, herpesviruses, and others. Public health responses involve vaccination, antiviral therapies, and infection-control measures. Taxonomy is overseen by the International Committee on Taxonomy of Viruses (ICTV), which classifies viruses by genome type, replication strategy, morphology, and host range. Viruses also contribute to scientific advances, such as viral vectors in gene therapy and systems biology studies. The discovery of viruses began with plant viruses in the late 19th century, followed by advances in microscopy and molecular biology that clarified their nature and diversity.