Tobacco mosaic virus
Ivanovsky and Beijerinck
The discovery of the tobacco mosaic virus established the existence of infectious agents smaller than bacteria and dependent on living hosts for replication. Ivanovsky (1892) demonstrated filterability; Beijerinck (1898) added host-dependent multiplication and coined 'virus'. Neither observed the virus itself. Their work founded virology — but also embedded a flawed 'fluid' model that delayed recognition of viruses as particulate, genetic entities.