the opposite side of the street L. These three houses contained fifteen inmates, all of them coming into frequent contact with the sick. These people were supplied with water from the well at v, and were throughout the epidemic exempt from the disease. Thus, to sum up, all the families that were supplied from the Schmidt well after the 20th of September had one or more members attacked by the fever; all the families that were furnished with water from other sources escaped. The conclusion is reasonable that in the drinking-water we have the means of the diffusion of the germs.
I have here shown a direct connection between the case of Otto Schmidt and that of every other infected person. This connection, traced through the drinking-water, was a discovery in our own epidemic; but it is by no means a discovery as a means of diffusion of the disease. This has long been known, and the literature of the subject is full of instruction.
It must be borne in mind, however, that not all water contaminated with organic impurities will cause typhoid fever. There must be among these impurities the specific poison of the disease. And this, as we have been able to prove, can be traced to former foci of typhoid fever. Other fluids may be contaminated in the same manner. An interesting report of an epidemic of the disease in England was given in the "Lancet" several years ago. A pasture had been manured by sewer products. Here were fed a large number of milch cows. In a neighboring town typhoid fever became rife. The victims, it was discovered, all used milk from this dairy; those who did not make use of it escaped. One can not help speculating about the way in which the disease-germs entered the milk. The more probable explanation is a mechanical one: that the germs adhered to the udders of the cows and were brushed off in the act of milking. It hardly seems reasonable that the germs passed through the organism of the cows and thus entered the milk. The theory of a contagium vivum is further fortified by the circumstance that, by boiling, the power to harm is taken away from the infected fluids.
It must be observed that between the entrance of the infecting elements into the system and the breaking out of the symptoms which define the disease, a varying period of time intervenes. This is called the period of incubation. It is a feature which distinguishes the entire group of infectious diseases. In determining the length of this period there are two elements of uncertainty: first, it is difficult to fix the exact time of infection; secondly, it is often impossible to define the exact beginning of the disease. In typhoid fever, especially, the onset is insidious and slow. Days and even weeks may be passed in a depressed, languid state of the health, before the disease is recognized as typhoid fever. Among business men these initial symptoms are often explained by taking cold, or by overwork. During the epidemic at Basle a few persons were attacked after a residence in the city of from seven to fourteen days; others after sixteen days. Haegler found in
