Date of origin: A very long time ago.
Author/inventor/context: First detected and described for biological systems in 1892 by Dmitri Ivanovsky.

A virus is an infectious agent that can only replicate inside a host. Viruses can infect all types of organisms and are found in almost every ecosystem on Earth. Unlike all other biological systems, viruses do not have cells that divide but replicate via infected host cells. Viruses play a fundamental role in evolution - the ability to resist viral attack has sculpted the genomes of all known organisms, from bacteria to humans. Since 1984 the term “virus” is also used to describe a malicious or unwanted computer programme used to gain access to, explore or damage computer systems.