For nearly seven decades since their discovery in 1898, viruses were the only organisms at the boundary between the living and the non-living. Their obligate host dependence, parasitism, and small genome sizes collectively made sure they weren’t classified as ‘life’ per se.
Each virion is composed of a nucleic acid (DNA or RNA) core that serves as the genetic material, surrounded by a protein coat, and, in some cases, a lipid (fat) layer outside that coat. Viruses’ life cycle is simple: they infect a host cell, use the cell’s machinery to make more copies of themselves, then infect a new cell to repeat the cycle.
