Viruses outnumber every other biological entity on Earth
Viruses infect every domain of life and inhabit nearly every ecosystem, yet they replicate only inside living cells. Since tobacco mosaic virus was found in the 1890s, more than 16,000 of millions of species have been described in detail. Outside a host they travel as virions—genome, protein capsid, sometimes a lipid envelope.
A virion packs DNA or RNA that encodes viral proteins, wrapped in a capsid and sometimes a lipid coat. Shapes run from simple helices and icosahedra to elaborate architectures; most particles are about one-hundredth the size of typical bacteria and invisible by ordinary light microscopy. An infected cell may be forced to spit out thousands of copies. Whether viruses count as life remains contested: they carry genes, evolve by selection, and reproduce, yet lack cellular structure and their own metabolism—hence phrases like “organisms at the edge of life.”
Origins are unclear because viruses leave no classic fossils. Three long-standing ideas compete: degeneration from once-independent cells (like today’s obligate intracellular bacteria), escape of genetic bits such as plasmids or transposons, and a virus-first world of protein–nucleic-acid complexes coevolving with early cells. Modern work treats viruses as ancient, predating the split into three domains of life, and suggests they arose more than once. Horizontal gene transfer via viruses boosts genetic diversity much as sex does.
Dmitri Ivanovsky showed in 1892 that filtered tobacco-plant sap still infected healthy plants; Martinus Beijerinck named the agent a virus in 1898, founding virology. Transmission routes include insect vectors, airborne coughs and sneezes (influenza, SARS-CoV-2, measles), faecal–oral spread (norovirus can infect with fewer than 100 particles), and blood or sexual contact (HIV). Animal hosts usually clear infection with immunity that vaccines can also train; chronic viruses such as HIV, HPV, and hepatitis evade those defenses. Roughly 100 antiviral drugs or combinations are used worldwide. Patrick Forterre’s virocell model calls the infected cell the living form and the virion a spore-like stage.
Source: Virus
