BIOLOGY AND MEDICINE—CHANDLER 321 
ideas didn’t make much headway. It was Semmelweis of Vienna who 
finally dealt the death blow to childbed fever as an epidemic occur- 
rence, and proved that even an eminent gentleman’s hands are not 
always clean. It is within the memory of many in this audience that 
aseptic surgery finally supplanted Lister’s heroic antiseptic measures, 
and that surgeons began paying more attention to washing their hands 
before an operation than after it. 
Some 20 years after Pasteur’s demonstrations of the germ cause of 
disease and the final putting to rest of the theory of spontaneous gen- 
eration, Robert Koch developed technical methods that made possible 
the easy isolation and study of particular kinds of germs, and then 
discovery followed discovery with almost incredible rapidity. In the 
short space of 15 or 20 years the causes of the majority of infectious 
diseases of man and animals were isolated and studied. The elusive 
and rather mysterious agents of disease that we call viruses, however, 
had to wait much longer for biologists and chemists to pry into their 
private affairs, and it is only now that very much progress is being 
made. 
An infectious disease is, however, an extremely complicated phe- 
nomenon. The interaction of a parasite and its host is not a static 
thing like the interaction of one chemical with another, capable of 
simple description, and following a well-defined course. We may be 
too prone to think, because we know what organism causes a disease 
and something about its biology, that we understand the disease it 
causes. Nothing could be farther from the truth. We are dealing 
with the interaction of two organisms both of which are capable of 
an amazing degree of adaptation to changing conditions. Every 
change or adaptation in one entails further adaptations in the other. 
A disease may be compared with an organism—it is born, it grows, it 
adapts itself to environment, and it finally dies. During its life it is 
influenced by a host of environmental factors which may profoundly 
alter its course. 
An infectious disease depends on the presence of a specific invading 
organism, but this may be only one of the necessary requisites. In 
almost every epidemic the number of healthy carriers—people who 
temporarily acquire a colony of the germs but show no evidence of 
disease—far exceeds the number of cases. In an epidemic of cerebro- 
spinal meningitis healthy carriers of the organism that we say causes 
it may outnumber the clinical cases 20 to 1. In most epidemics of 
such diseases as diphtheria, whooping cough, dysentery, and even 
cholera, the ratio is from 5 or 10 to 1. 
If disease develops in only one-fifth to one-twentieth of the people 
reached by a’particular pathogenic germ, it is evident that there are 
other factors playing very important roles in its production. Among 
these are a proper balance of the glands of internal secretion, good 
