Viruse

Viruse are non-cellular infectious agents that depend on host cells to replicate. They range in size from about 20 to 300 nanometers and contain genetic material—DNA or RNA—enclosed in a protein coat called a capsid. Some viruse also have a lipid envelope derived from the host cell membrane. Inside the host, viruse direct the cell's machinery to produce viral components, often disrupting normal cell function.

Viruse are classified by genome type (DNA or RNA), strandedness (single or double), symmetry of the capsid, and presence of an outer envelope. The Baltimore classification system groups viruses into seven categories based on replication strategy. Reproduction is entirely dependent on host enzymes; viruse do not carry their own metabolic systems.

Life cycle: attachment to a suitable host receptor, entry by endocytosis or membrane fusion, genome release, transcription and replication, assembly of new virions, and release via lysis or budding. Some viruse integrate into the host genome (as proviruses) or establish persistent infections.

Host range varies widely—viruse can infect animals, plants, fungi, or bacteria (bacteriophages). Transmission routes include respiratory droplets, bodily fluids, ingestion, vectors, or environmental reservoirs. Most viruse do not cause disease in every infection; virulence depends on viral factors and host response. Prevention and treatment include vaccines, antiviral medicines that inhibit replication, supportive care, and public health measures.

History: the existence of viruse was established in the late 19th and early 20th centuries with work by scientists like Ivanovsky and Beijerinck, who showed viruses could pass through filters that blocked cells. Modern virology studies their role in ecology and medicine. In ecology, viruse regulate populations and drive gene transfer among microbes.