From the middle until nearly the close of the nineteenth century, the germ theory, during the period when it was little else than a theory, furnished many arguments for those who contended that filth was a fertile source of disease. Putrefaction and fermentation were known to be similar processes, and were believed to be due to the vital activity of minute organisms. There were good grounds for believing that diseases of an infectious nature were also dependent on the growth in the body of similar 'germs,' and this theory from 1850 grew rapidly into favor. The germ theory led Dr. Farr, Registrar General of England, to classify most of the infectious diseases as 'zymotic' or fermentative diseases, for the disease poison was supposed to act, as in truth it does, as a ferment in the blood or other tissues in the body. If both putrefaction and disease were due to the action of minute organisms, what more reasonable than to believe, said the theorists, that putrefying material harbored and developed the 'germs' of disease?
The filth theory then, which has had such a powerful influence on the public mind, assumed that most of the infectious diseases were directly and specifically caused by germs or other more subtle emanations from decaying animal or vegetable matter.
Furthermore, it was claimed that while such emanations might not in every case produce a specific disease, they did tend almost always to affect injuriously the general health, and lower the vitality of persons habitually exposed. Hence the sewer gas theory which has found such acceptance, and which has taught that the gas formed from the filth in drains is so injurious to human life that portions so minute as not to be appreciated by the senses are yet harmful in the extreme. It was taught by medical men and health officials that filth and decay in every form were a serious menace to health, both from the disease germs which they contain, and the poisonous gases which they give off; and this teaching is received and accepted, even to-day, by a large portion of the medical profession, health officers and the public at large.
It is true that ever since this theory was promulgated some have been led to doubt its dicta, because in the first place they often found filth to abound where little zymotic disease existed, and even where the 'general health' of the people was high. On the other hand, zymotic diseases were frequently found in the cleanest of dwellings, and where the best of plumbing kept out all sewer gas. But most sanitary officials accepted the theory as fact, and acted accordingly, some used it simply as a working theory awaiting more definite knowledge, and a few were led by their experience to allow it little weight in their work.
As soon as the germ theory of disease ceased to be a mere theory, and the true facts in regard to the etiology of the infectious diseases began to be known, and bacteriology gave us exact knowledge of the