Passing infectious secretion through filters with the tiniest pore size demonstrated that infectious particles far smaller than bacteria could pass through these filters and satisfy Koch's postulates. In 1903 Negri demonstrated eosinophilic cytoplasmic inclusion in infected neurons of rabies victims. 50 years later these were shown to be composed of viral particles and viral proteins. These lesions now known as Negri bodies are pathognomonic. Because viruses are obligate intracellular parasites initial propagation of viruses used whole animals such as dogs and monkeys.
Tissue culture is made from a cell line transformed by a tumor.
One famous line is called HeLa, derived from the cervical carcinoma of a woman named Helen Lane. These cells are grown in a dish as a monolayer in nutrient media in the laboratory and split into single cells with enzymes such as trypsin and then the trypsinized cells are placed on new plates where they divide and grow forming a new monolayer of cells. The development of nutrient media was essential and Harry Eagle of this institution was instrumental in providing the ingredients needed for cells to grow in culture. The development of antibiotics at the same time allowed cell lines to survive by preventing overgrowth of the cells by bacteria. Viral infection of cells in culture is followed using the light microscope which demonstrates when the cell line has been altered.