A virus is a small infectious agent composed of genetic material (DNA or RNA) encased in a protective protein shell called a capsid and, in many cases, a lipid envelope derived from host membranes. Unlike cellular organisms, viruses lack metabolic machinery and replicate only within living host cells.
Structure, classification and replication
Viruses exhibit a wide range of sizes and shapes, from helical rods and icosahedral particles to complex bacteriophages with tails. The viral genome may be single- or double-stranded, linear or circular, and composed of either RNA or DNA, which forms the basis of the Baltimore classification system. Surrounding the genome is a capsid built from repeating protein subunits called capsomers. Enveloped viruses, such as influenza and HIV, acquire a lipid bilayer from the host cell membrane that is studded with viral glycoproteins required for attachment and entry. The viral life cycle begins when a virion attaches to specific receptors on a host cell and enters via membrane fusion, receptor-mediated endocytosis or penetration. After uncoating, the genome directs synthesis of viral proteins and replication of new genomes using host ribosomes and, in some cases, virus-encoded polymerases or reverse transcriptase. Newly formed nucleocapsids assemble and exit the cell by budding, exocytosis or cell lysis. Because viruses rely on host machinery, they are obligate intracellular parasites and are not considered living organisms in the classical sense.
Diversity and impact on health and biotechnology
Viruses infect every domain of life, including animals, plants, fungi and bacteria. Human viral pathogens cause a spectrum of diseases: respiratory viruses like influenza and SARS-CoV-2 spread through droplets and aerosols; blood-borne viruses such as HIV and hepatitis viruses lead to chronic infections; neurotropic viruses like rabies cause encephalitis; and childhood viruses like measles can suppress immune function. In agriculture, plant viruses reduce crop yields and are transmitted by insect vectors. Bacteriophages infect bacteria and play major roles in controlling microbial populations and driving gene exchange in ecosystems; phage therapy is being explored to combat antibiotic-resistant bacteria. Viruses also serve as tools in molecular biology and medicine: adenoviral and lentiviral vectors deliver genes in gene therapy, and virus-like particles are used in vaccines. Studying viral replication and evolution has led to discoveries such as reverse transcriptase and CRISPR antiviral defence systems.
Viruses are ubiquitous and diverse, shaping the evolution of their hosts and influencing ecological and health outcomes. Understanding their structure and replication informs prevention, antiviral drug development and the harnessing of viruses as research and therapeutic tools.
Related Terms: Virion, Virology, Pathogen, Bacteriophage, Envelope