CAUSING ACUTE INFECTIONS. 89 
in the body of the bacterial cells and set free only after 
their death. It is possible to immunize animals against 
cholera by injecting small amounts of the killed culture 
or very small doses of the living organisms. The 
blood-serum of animals immunized in this way con¬ 
tains substances that dissolve the spirilla-bacteriolysins, 
and substances that clump them—agglutinins. The 
agglutinins are made use of in diagnosing cholera in 
the same way as in the diagnosis of typhoid fever (see 
Widal reaction). Human beings that have recovered 
from cholera are immune to the disease, but they re¬ 
main so only for a few months. Efforts to protect 
human beings by injecting the killed cultures have been 
made in India on a large scale, but the results have 
been only partially successful. 
The Bacillus of Diphtheria. 
Diphtheria is an infectious disease caused by the 
diphtheria bacillus, sometimes called the Klebs-Loffler 
bacillus, after the two men who discovered it. The 
word diphtheria is derived from a Greek word mean¬ 
ing a membrane, because of the characteristic false 
membrane that forms in the throat. The bacillus 
causes infection most frequently in the throat or nose, 
although it may grow on the gums or about the teeth. 
It is possible for diphtheria bacilli to cause infection 
of the middle ear, the sinuses of the nose, and the lung 
(pneumonia). Rarely it extends to the skin about the 
mouth, or to the genitalia or rectum. The diphtheria 
bacillus is one of the few types that can be identified 
by its appearance under the microscope because its 
Immunity- 
Morphol¬ 
ogy 
