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Viruses Filterable Agents of DiseaseThey have co-evolved with and influenced virtually all life forms. And yet we've only known about viruses for the past 125 years. I'm Jim Metzner and this is the Pulse of the PlanetMcDonald: So they've been around for millennia, and it's only very recently that humans have actually discovered their existence.Virologist Sarah McDonald.McDonald: One of the earliest experiments that described what viruses are is a classic experiment in which a scientist discovered that what was the causative agent of a plant disease was actually what we call a "filterable agent". A filterable agent is essentially a germ, if you will, that is able to pass through a very small filter that would otherwise remove larger germs, including bacteria and parasites.In 1892, Russian biologist Dmitri Ivanovsky performed an experiment that led to the discovery of viruses.McDonald: The experiment that he performed was essentially to take a sick tobacco plant and to crush it up and then he took the tobacco plant juice from the sick tobacco plant and he passed it through a filter. This filter is designed to remove larger germs, such as bacteria and parasites. Then he took this filtered juice or the plant debris, and he individually put each of those preparations onto a healthy plant. He was shocked to find that it was the filtered juice that caused the healthy plant to get sick and not the debris. This proved that what actually was causing the plants to get sick was something smaller then a bacteria. This is the first description of a virus as a filterable agent of disease.We'll hear more on viruses in future programs. Pulse of the Planet is made possible in part by Virginia Tech, inventing the future through a hands-on approach to education and research.