CONTROL OF VIRUS DISEASES 83 
virus is that of immunization against yellow fever. It was first shown 
by Theiler in 1930 that the yellow fever virus could be altered by 
passage of mouse brains and this was followed up by the further 
modification in chick embryo tissue culture. Eventually, a strain of 
yellow fever virus, known as 17D, was evolved which was satis- 
factory for human inoculation and this has been much used. Indeed, 
during the year ending June, 1943, the Rockefeller Foundation 
distributed about seven and a half million doses of this vaccine. 
Another virus disease which can now be treated with a modified 
but active vaccine is the mosquito-borne dengue or “break-bone’”’ 
fever. Large-scale experiments were carried out in the U.S.A. during 
the late war and although great difficulty was encountered in finding a 
susceptible animal, the virus was eventually transmitted to a particular 
strain of white mice. This was only achieved after the virus had been 
concentrated by means of the ultracentrifuge. After repeated passage 
of many generations of mice, a strain of virus was evolved which 
produced only the skin rash, which is characteristic of dengue, without 
the fever and other symptoms. This strain was effective, however, in 
stimulating the formation of antibodies against the severe form of the 
disease as well. 
The artificial modification of influenza virus to produce a vaccine 
is one of the most striking developments in the control of virus 
diseases. It began with the work of Burnet (1936) who found that a 
strain of the virus, propagated in ferrets, would multiply on the hen’s 
egg membrane but gave no visible disease in the embryo. That it 
was multiplying could be shown by inoculation back to ferrets. 
However, with continued passage through eggs the virus began to 
produce visible pocks or lesions in the egg membrane. More passages 
led to an increase of virulence and the virus commenced to invade 
the chick embryo. Simultaneously with this increase of virulence for 
the chick embryo, there occurred a decrease in virulence for ferrets, 
mice, and men. The production of this egg culture type of influenza 
vaccine was carried out on a large scale in the U.S.A. during the late 
war to meet a possible recurrence of an influenza pandemic such as 
occurred in 1919 after the first world war. Two methods of preparing 
the vaccine were used. In one the virus was purified and concentrated 
by precipitation with chicken red blood cells. It was then removed 
from the blood cells by raising the temperature and low speed centri- 
fugation to sediment the blood cells. The virus was then inactivated 
by formaldehyde. 
