(August 2004) Viruses were first described as agents that were not retained by the unglazed porcelain filters that early researchers used to remove bacteria from solution. In 1898, scientists observed that the agents responsible for foot and mouth disease, as well as tobacco mosaic disease, were filterable. It was discovered that these agents would not grow unless cells were added to the growth medium. Serial dilutions of these infectious agents were performed to rule out toxins as the cause of symptoms. The failure of these pathogens to multiply in solutions in which bacteria were able to grow, as well as…