The influenza pandemic of 1918–1919 dramatically altered biomedical knowledge of the disease. At its onset, the foundation of scientific knowledge was information collected during the previous major pandemic of 1889–1890. The work of Otto Leichtenstern, first published in 1896, described the major epidemiological and pathological features of pandemic influenza and was cited extensively over the next two decades. Richard Pfeiffer announced in 1892 and 1893 that he had discovered influenza's cause. Pfeiffer's bacillus (Bacillus influenzae) was a major focus of attention and some controversy between 1892 and 1920. The role this organism or these organisms played in influenza dominated medical discussion during the great pandemic. Many vaccines were developed and used during the 1918–1919 pandemic. The medical literature was full of contradictory claims of their success; there was apparently no consensus on how to judge the reported results of these vaccine trials. The result of the vaccine controversy was both a further waning of confidence in Pfeiffer's bacillus as the agent of influenza and the emergence of an early set of criteria for valid vaccine trials.