Physicians and patients have always had a great fascination with the invisible pathogen. Koch’s criteria,1 set out as early as 1882, have generally been ignored, that is, a microorganism must be (1) present in every case of the disease and (2) isolated from the host with the disease and grown in pure culture, (3) the specific disease must be reproduced when a pure culture of the microbe is inoculated into a healthy susceptible host, and (4) the pathogen must be recoverable from the experimentally infected host. In 1892, when various bacteria had already been identified, Dimitrii Ivanovsky demonstrated that an infectious agent underlying tobacco mosaic virus disease was small enough to pass through a filter that trapped all known pathogens. Based on this method, which has to be considered cutting edge for that period, a human illness with an unknown etiology was believed to be caused by an unknown blood poison: multiple sclerosis (MS). The Greek word for poison is virus, and when the invention of the electron microscope allowed the visualization of viruses in the 1930s, modern virology was born.
Stüve O, Racke M, Hemmer B. Viral Pathogens in Multiple SclerosisAn Intriguing (Hi)story. Arch Neurol. 2004;61(10):1500-1502. doi:10.1001/archneur.61.10.1500