No department of medicine has been cultivated in recent years with such zeal and with such fruitful results, as that relating to the causes of infectious diseases. The most important of these results for preventive medicine, and for the welfare of mankind is the knowledge that a large proportion of the causes of sickness and death are removable.
It is evident that efforts to preserve health will be most intelligently and effectually applied when they are based upon an accurate and full knowledge of the agencies which cause disease. Public and private hygiene, however, can not, and fortunately has not, waited for the full light of that day whose dawn has only begun to appear, when we shall have a clear insight into the causation of preventable diseases. Cleanliness and comfort demand that means shall be taken to render pure the ground on which we live, the air which we