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243 
our stay agreeable as well as scientifically successful. We wish also to 
extend our thanks to the members of the laboratory staff, especially to Dr. 
Wm. E. Musgrave, Pathologist, Dr. Wm. B. Wherry, Bacteriologist, and 
Dr. Maximilian Herzog, Pathologist, for many favors. 
The constant supply of fresh variola virus requisite for our work was 
secured through the kindness of Maj. E. C. Carter, Commissioner of 
Public Health, and of Dr. V. G. Heiser, Chief of the Quarantine Service. 
A constant supply of fresh vaccine virus was given us by Dr. Paul G. 
Woolley, chief of the Serum Laboratory, Bureau of Science. Dr. W. R. 
Moulden, resident physician at Bilibid Prison, and Dr. H. B. Wilkin- 
son, resident physician at the San Lazaro Hospital, gave us opportunity 
to study the clinical material under their control. 
The greater part of the funds which made the expedition possible was 
generously supplied by Mr. Augustus Hemenway, Dr. John C. Phillips, 
and Dr. Wm. L. Richardson. The Rockefeller Institute for Medical 
Research aided the expedition by appointing one of the workers to a 
research fellowship. _ = 
If material taken from a smallpox lesion on man be inoculated on an 
epithelial surface of a calf, after a definite period, a lesion which anatom- 
ically closely resembles the parent lesion, the pock is produced. Its ap- 
pearance is accompanied by swelling of the nearest lymph nodes, fever and 
constitutional disturbance. After the process has subsided there is im- 
munity to further inoculation. The material from the lesion, transferred 
to an epithelial surface on another calf, produces a similar result, and 
after a series of transfers from animal to animal may be returned to man, 
and it develops not the original disease, smallpox, but the incomparably 
milder disease, vaccinia. 
Many of the strains of vaccine virus now used are known to have been 
derived from smallpox and we are justified in believing that all strains 
were originally so derived. Just how many transfers from animal to ani- 
mal is necessary before the virus loses its power to produce smallpox is not 
known. One of our experiments in this regard is interesting. The con- 
tents of a smallpox vesicle in a monkey was used to inoculate the cornea 
of a rabbit. After 5 successful transfers to other rabbits the virus was 
used to inoculate a monkey and not vaccinia, but smallpox was produced. 
The disease vaccinia confers immunity not only against vaccinia but 
against smallpox. The immunity, although not absolute, is stronger than 
is developed by most other infectious diseases. Vaccinia differs from 
smallpox in three striking respects: 
First. The period of incubation is shorter, being in man 5 days. The 
incubation period of smallpox is 12 days. 
Second. In vaccinia there is no general exanthem. There may be a 
few vesicles around the point of inoculation, but they develop simulta- 
neously with and not after the main lesion and are probably due to a 
distribution of the virus at the time of inoculation. 
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