322 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1944 
nutrition, especially with respect to vitamins, and the development 
of specific immunity or resistance. There can be no doubt that these 
same factors play a large part in determining the course and outcome 
of a disease after it has gotten a start. A physician, then, if he is to 
make the most of his effort to help in suppressing disease, must be far 
more than a dispenser of medicine. He must, indeed, be familiar with 
more phases of biology than are most biologists. He must understand 
anatomy, histology, general physiology, endocrinology, embryology, 
psychology, nutrition, immunology, and even genetics in order to have 
a proper understanding of his patient, and he must be a bacteriologist 
or parasitologist to understand the capabilities and vulnerabilities of 
the invading organism. 
Some relations of heredity and genetics to disease have been known 
for a long time, but more progress has been made in genetic control of 
disease in plants and even in domestic animals than in man. Effects 
of genetic constitution of human beings on the course of disease and 
development of resistance are still very little understood, and still less 
is known about effects of genetic constitution of pathogenic organisms 
and means of altering it. Herein lies an almost untouched field with 
vast possibilities for the future. 
Experimental breeding of mice has resulted in decreasing torte 
from a particular disease from 82 to 24 percent in six generations, and 
to 8 percent over a period of years. In six generations of chickens 
mortality from fowl typhoid decreased from 85 to 10 percent. Recent 
studies indicate that alterations in genetic constitution comparable to 
mutations in insects and plants occur also in bacteria and even in 
viruses. In a period of a few hours many kinds of bacteria and viruses 
may reproduce in such numbers that if their rate of mutation is com- 
parable with that thought true for fruitflies, each gene the bacteria 
possesses should mutate at least once. With even slightly favorable 
selection, replacement of the parent population by mutants is possible 
in short periods of time. 
Viruses have many characteristics of genes, differing principally in 
their ability to move from cell to cell. There is evidence that the 
mutation of viruses is comparable with mutation of genes. The de- 
velopment of relatively nonpathogenic varieties of viruses or bacteria 
is the real basis for the production of effective vaccines against such 
diseases as smallpox and yellow fever, and probably for the rise and 
fall of epidemics of cholera and diphtheria. It has recently been 
discovered that the virus of infantile paralysis genetically altered by 
mouse adaptation, when mixed with the parent virus, has great power 
to protect monkeys from paralysis. What causes the protection is not 
yet known, but the result of this basic discovery may be very far 
reaching. 
