The wonderful increase in our knowledge of the causation of many diseases is one of the most marked features of recent years, and although very much indeed has yet to be learnt, sufficient is known to admit of practical application with beneficial results. The germ theory of disease is no longer regarded merely as a plausible hypothesis, but is accepted as a fact which is becoming every day more significant. "Find the microbe" is the key to as many enigmas as "Cherchez la femme"

The aim of sanitary science must be, as far as possible, to render our buildings and their appurtenances free from such conditions as contribute to the growth and distribution of pathogenic organisms, and to provide those conditions under which such organisms cannot exist. The knowledge which has been gained shows clearly that the solution of this problem will not be easy. Dr. Sidney Martin's experiments with the typhoid bacillus, for example, prove that it soon dies in "virgin" soils, whether these are sandy or peaty, but that it rapidly multiplies in moist sterilised samples of soils from gardens and other cultivated places, and retains its viability and vegetative properties for months and perhaps years. Moist sterilised soil which had been impregnated with the typhoid bacillus was found to contain the bacillus at the end of 456 days. The soil was then naturally dried for forty-nine days till it "could be readily powdered into a fine dust;" the bacillus was still there.

Other experiments showed that the bacillus spread through the soil in different directions.

Experiments with sterilised soils are, however, of little practical use. It is more important to know how long the typhoid bacillus can survive in soils containing other bacteria. No definite statement on this point can yet be made, but there is reason to believe that, while in many cases the typhoid bacillus may be overcome by the other bacteria in the general struggle for existence, in some cases it will live and retain its dangerous properties for months. In a report, written in 1899, on "Enteric Fever in the City of Chichester," Dr. Theodore Thomson and Col. J. T. Marsh, R.E., came to the conclusion that the most probable cause of the repeated recurrence of enteric (typhoid) fever in that city was the foul condition of the soil, due to the "leaky cesspools and cesspit privies, which, until some three years ago, formed the sole methods of disposal of all its excrementitious matters."

The lessons to be learnt from these and other investigations are that some pathogenic organisms are not by any means so short-lived as is commonly supposed, that they may spread rapidly under suitable conditions, and that they may exist for a considerable time even in dust so dry as to be blown about by the wind. The degree of temperature at which most rapid growth occurs differs, but most bacteria develop at the temperatures which are ordinarily maintained in inhabited buildings, while some (among which the typhoid bacillus is unfortunately found) can, in the words of Dr. Newman, "withstand freezing for weeks."