Haemophilus influenzae is a leading cause of acute bacterial infections in infancy and childhood and an important agent of chronic pulmonary infections in children and adults. The bacillus was so named because it was observed in the purulent sputum of individuals stricken with influenzal infection in the European outbreak of 1889–1892 (Pfeiffer, 1892). The erroneous conclusion that this bacterium was the cause of influenza prevailed until the studies of Smith et al. (1933) established beyond doubt that influenza is caused by viruses. By that time, however, its nomenclature had been firmly established.