A tobacco disease slipped through a filter fine enough to catch every bacterium
In 1892, Dmitri Ivanovsky filtered infectious sap from diseased tobacco plants through porcelain fine enough to trap every known bacterium, and it still caused disease. Martinus Beijerinck repeated the test in 1898, showed the agent could multiply inside living cells, and coined the word 'virus' for it — the first evidence of a pathogen smaller than any known microbe.