350 BACTERIOLOGY. 
contracting the disease. It is, therefore, apparent that 
infection in diphtheria, as in other infectious diseases, 
requires not only the presence of virulent bacilli, but 
also a susceptib lity to the disease, which may he 
inherited or acquired. Among the predisposing in- 
fluences which contribute to the production of diph- 
theritic infection may be mentioned the breathing of 
foul air and living in overcrowded and ill-ventilated 
rooms, poor food, certain diseases, more particularly 
catarrhal inflammations of the mucous membranes, and 
depressing conditions generally. Under these condi- 
tions an infected mucous membrane may become sus- 
ceptible to disease. In connection with Beebe (1894) 
I made an examination of the throats of 330 healthy 
persons who had not come in contact, so far as known, 
with diphtheria, and we found virulent bacilli in 8 only, 
2 of whom later developed the disease. In 24 of the 330 
healthy throats non-virulent bacilli or attenuated forms 
of the diphtheris™bacill us were found. Very similar 
observations have been made by others in many widely 
separated countries. 
The Persistence of Diphtheria Bacilli in the Throat. 
The continued presence of virulent diphtheria bacilli 
in the throats of patients who have recovered from the 
disease, and after the disappearance of the exudate, has 
been repeatedly demonstrated. Beebe and I found that 
in 304 of 605 consecutive cases the bacilli disappeared 
within three days after the disappearance of the pseudo- 
membrane; in 176 cases they persisted for seven days, 
in 64 cases for twelve days, in 36 cases for fifteen days, 
in 12 cases for three weeks, in 4 cases for four weeks, 
and in 2 cases for nine weeks. Since then I have met 
with a case in which they persisted for six months. 
4 
