 He observed that the agent multiplied only in cells that were dividing and he called it a "contagium vivum fluidum" (soluble living germ) and re-introduced the word "virus".Beijerinck maintained that viruses were liquid in nature, a theory later discredited by the American biochemist and virologist Wendell Meredith Stanley (1904–1971), who proved that they were in fact, particles.In the same year Friedrich Loeffler (1852–1915) and Paul Frosch (1860–1928) passed the first animal virus through a similar filter and discovered the cause of foot-and-mouth disease.