BACILLUS INFLUENZA 453 
sterile defibrinated pigeon or rabbit blood is added to 50 cc. of neutral 
nutrient broth. After incubation to demonstrate its sterility, the 
medium is ready for inoculation. Attempts to grow the bacilli on 
entirely hemoglobin-free media have been uniformly negative; no 
development occurs in ordinary media, unless as Jacoby and P'ranken- 
thal claim, histidine may be substituted. 
B. influenzae is an aerobic bacillus. It has not been grown in the 
absence of oxygen. Growth does not take place below 25° C. nor 
above 42° to 43° C. The optimum temperature is 37° C. It is possible 
to maintain cultures by transplanting them upon fresh hemoglobin 
media at intervals not greater than five days. Drying is rapidly fatal 
to influenza bacilli; dried in mucus the organisms are not viable 
after one to three days. They may remain alive in moist mucus for 
nearly two weeks, however. Ten minutes' exposure at 57° C. kills 
them and ordinary chemical disinfectants, bichloride of mercury 1 to 
1000, and 5 per cent carbolic acid, destroy them in a few minutes. 
Fig. 65. — Influenza bacillus from sputum. X 1200. (KoUe and Hetsch.) 
Products of Growth.— The nature of the products of metabolism of 
the influenza bacillus are unknown. Enz'ymes have not been detected 
in cultures of the organism and soluble toxins have not been demon- 
strated. There is evidence that the cell substance of the bacilli is 
toxic; if is probable that this toxic substance is endotoxic in character. 
The Etiology of Influenza.— The highly infectious disease known as 
influenza, sometimes called "la Grippe," or "flu," appears pandemi- 
cally at infrequent intervals. It occurs more commonly in the colder 
months. The recession of a pan epidemic is usually characterized by 
a rapidly decreasing succession of lesser waves of infection. During 
interepidemic periods, local or localized epidemics are of not infrequent 
occurrence. 
Invasion apparently takes place through the respiratory tract, 
usually by droplet infection, and frequently spreads by continuity to 
the lungs where a purulent broncho- or lobar pneumonia develops in 
