Still, even with these amazing break-throughs in disease prevention, none of the scientist of the time had any clue to what kind of 'monster' they were dealing with. In 1892, Russian Dmitri Ivanovski discovered the very first clue that set these microbes in a class of there own. Even though he was not in the practice of studying human diseases, Ivanovski gave us the first proof that viruses so exist. Ivanovski's main research included the tobacco mosaic disease. Using special filters Ivanovski attempted to separate out the bacteria that was causing the infection. To his dismay, even after several iterations of the filtering process and exposing it to alcohol and fermalin, the tobacco plants continued to become infected and die.
Six years later, a Dutch botanist, Martinus Biejerinck performed a similar experiment. However, he had not read about Ivanovski's work since it was only published in not very well known Russian journals. He performed the same filtering method, but he took it a step further. Though the filtering method might have removed the bacteria, it might not have removed toxins created by bacteria. These toxins could also cause diseases. To see if it was the toxins, he infected a healthy plant and then tried to infect another plant with fluid from the now infected healthy plant. If it was toxins, the next plant would not be infected. It did however, telling Biejerinck that it wasn't toxins. Trying something different, he let the sap from an infected plant sit for three months and tried to infect a healthy plant. It still infected. He tried adding alcohol and formalin which would be enough to kill microorganisms. It did nothing to prevent the fluid from infecting again. So what was causing this disease in these plants if it wasn't toxins or bacteria? Perhaps it was bacterial spores? They could pass through the filters. To test this, Biejerinck heated the fluid to ninety degrees centigrade. All of a sudden, the fluid stopped infecting the plants! However, this didn't prove it was bacterial spores. Quite the contrary. It takes a hundred degrees centigrade to kill the spores, not ninety. This was something else completely.
Through their research, Biejernick and Ivanovski had discovered a new disease-causing agent. Biejernick believed this agent to be a fluid, he called it a "contagious living fluid." Later this liquid was renamed 'virus' for the Latin word poison.