328 |= ANNUAL REPORT SMITHSONIAN INSTITUTION, 1944 
not until the end of the nineteenth century that a basis for the pre- 
vention of malaria was discovered. 
A few practices of primitive people suggest attempts, probably 
unwitting, to prevent disease. In India, for instance, I found a 
primitive tribe, the Santals, who never drink water directly from a 
stream or pond, but from a little hole in the sand a foot or.so away, 
thus practicing sand filtration, one of the prime tools of modern san- 
itary engineering. The unfitness of natural water for drinking was 
recognized long ago. Cyrus of Persia carried boiled water for his 
troops 2,500 years ago. The low repute of water as a beverage even 
in the unenlightened middle ages is evidenced by a thirteenth-century 
writer who, describing the extreme poverty of Franciscan monks who 
settled in London in 1224, exclaimed, “I have seen the brothers drink 
ale so sour that some would have preferred to drink water.” The 
head-hunting, carrion-eating Nagas of the Assam hills drink only a 
rice beer, carrying starters with them when they go on trips. 
Preventive medicine as practiced at present has three principal legs 
to rest on: (1) the upkeep of natural resistance by general hygienic 
measures, including a proper hormone balance and optimum nutrition ; 
(2) the artificial stimulation of specific immunity or resistance; and 
(3) protection against access of disease germs via water, food, air, 
or insect transmission. 
The general principles involved in the first of these have been known 
for a long time, but the details have only recently been filled in by 
the discoveries with respect to hormones, minerals, and vitamins that 
I have already mentioned. I have already called your attention to 
the fact that in an epidemic only a small percentage of the individuals 
that are actually exposed develop a disease. The determining factors 
are the dosage of germs that gain access to an individual, and the 
natural resistance he has. The higher the natural resistance, the 
greater the dosage he can withstand. 
The second leg on which preventive medicine rests, artificial stimu- 
lation of immunity, I have already discussed. On it we depend very 
largely for our protection against smallpox, diphtheria, tetanus, rabies, 
yellow fever, spotted fever, typhoid fever, and many other diseases. 
The third leg on which preventive medicine rests — protection 
against dissemination of germs—I have so far said little about, but 
here enormous strides have been made within a short space of time. 
Famous in sanitary history is the case of the Broad Street pump in 
London in 1854, around which centered an explosive outbreak of 
cholera. After everything from the chemical nature of the soil to 
dust bins in cellars had been investigated, the relationship between 
drinking water from the well and attacks of cholera became clear. 
Nature had provided a grim lesson out of which grew modern sani- 
tary engineering. In the intervening 90 years modern water purifi- 
