The nature of viruses has been a matter for speculation since Iwanowski1 demonstrated in 1892 that the infective agent causing the mosaic disease of tobacco plants could pass through a porcelain filter. This was the first known virus. Iwanowski's work was confirmed independently by Beijerinck.2 Loeffler and Frosch3 in 1898 showed that the causative agent of foot and mouth disease also is filtrable through a porcelain candle. Since then more than a hundred filtrable agents belonging to the class now known as viruses have been demonstrated to cause disease in plants and animals.
Many difficulties beset the efforts of investigators to determine the nature of viruses. Some have held that they are living organisms; in view of the fact that viruses do not propagate except in the presence of living tissue and because of their behavior in other respects, some have suggested that these infective agents are