When were viruses discovered?

Viruses were identified as non-bacterial toxic agents by many researchers beginning with Louis Pasteur in the 1870's, Charles Chamberland in 1884, and Dimitri Ivanovsky in 1892. Viruses were different because they could penetrate filters that stopped bacteria. Martinus Beijerinck defined the virus as an extremely small nucleotide particle rather than a fluid in 1898. It was not until 1935 that WenDELL Stanley was able to crystallize viral particles, and they were imaged for the first time using an electron microscope in 1939.