The bacillus of influenza was discovered and accurately described by Pfeiffer in 1892. Its etiological relationship to the disease is now beyond serious dispute. It is a comparatively minute bacillus, Gram-negative, and devoid of capsule or spores. The chief seat of infection is the respiratory tract. The bacillus requires very special conditions for its artificial culture. The bacteriological text-books prescribe the use of fresh blood smears, and generally state that the organism can be grown along with other bacteria—that is to say, in symbiosis with them. In the laboratory of the Scottish Asylums it was ascertained that the bacillus of influenza has the remarkable property of being stimulated to growth by mere proximity to other bacteria, and thus the alternate drill method has come to be a routine of the laboratory whenever this bacillus has to be subcultured.