The bacillus is pathogenic for certain of the laboratory
animals, the guinea-pig in particular being subject to
fatal infection. The dose required to cause death of a
guinea-pig varies considerably, in the immunization ex-
periments of Deline and Kole1 -fa of a 24-hour old culture
being fatal in twenty-four hours. These scholars found
that the toxicity of the culture resides not in a soluble
toxin, but in the bodies of the bacilli. The outcome of
the researches, which were made most scientifically and
FIG. 127.—Bacillus of influenza; cover-glass preparation of sputum from a case
of influenza, showing the bacilli in leukocytes; highly magnified (Pfeiffer).
painstakingly, was the total failure to produce immunity.
Increasing doses of the cultures injected into the peri-
toneum resulted in enabling the animals to resist rather
more than a fatal dose, but never enabled them to main-
tain vitality when large doses were administered. This
discovery is in exact harmony with the familiar clinical
observation that, instead of an individual being immune
after an attack of influenza, he is as susceptible as before,,
if not more so.
1 Zeitschrift fur Hygiene, etc., Bd. xxiv., 1897, Heft. 2.