Final answer:
Filterable viruses were discovered after the development of the Chamberland-Pasteur filter, which removed visible bacteria from liquid samples. These filterable infectious agents were a new type of extremely small particle, different from bacteria.
Step-by-step explanation:
Filterable viruses were discovered after the development of a porcelain filter called the Chamberland-Pasteur filter. This filter could remove all visible bacteria from any liquid sample under a microscope. In 1892, Dmitri Ivanowski showed that tobacco mosaic disease could still be transmitted through liquid plant extracts even after the filter removed all viable bacteria. It was later proven that these filterable infectious agents were a new type of very small particle and not simply small bacteria.