The influenza bacillus was first isolated by Pfeiffer during an epidemic of 1889. The frequency of its occurrence led to the erroneous deduction that it was the cause of the disease. The modern presumption is that the epidemic was the work of a virus. Nonetheless, the concept that this bacterial organism was the primary agent led to the misleading name, Haemophilus influenzae, which it unfortunately retains today. During the last decade the primary pathogen in numerous epidemics of influenza of more or less limited scope has been shown to be a virus. However, extensive bacteriologic studies carried out during the 1918 influenza pandemic demonstrated the great importance of H. influenzae as a secondary invader. This evidence was completely reviewed by Jordan1 in his book "Epidemic Influenza." A number of investigators of this pandemic expressed the opinion that its unparalleled severity was due to the concurrent reappearance of the epidemic