THE demonstration of a few of the avenues by which infection is transmitted is among the triumphs of modern experimental medicine. By its revelations cholera is now known to be mainly a water-borne disease; likewise it is recognized that typhoid is transmitted by those means by which the waste products of an infected individual are transferred either directly or in round-about ways to the food of another; malaria is no longer thought to be wafted by the night air, but is known to be directly carried to and introduced into the system by the mosquitoes; and, even a later triumph, yellow fever is seen to approach its human victim through the same hosts; while, finally, it has been determined that bubonic plague, the scourge of the tropical east, is carried by the rat flea. Notwithstanding these recognized avenues of transmission in specific instances, many other and more common infections continue to travel from one to another by paths that we do not know.
Before the knowledge of cholera transmission by water, it would have been considered a scientific contribution to the subject to have demonstrated the absence of cholera germs in twenty-four samples of water taken at random some of which perhaps were dirty; but to-day we know that the bacteriological study of water for evidence of cholera will usually demonstrate the avenue of infection only when and where cholera is prevalent. Similarly, it would be a matter of the greatest surprise if the examinations of twenty-four or many more samples of water or food for typhoid germs revealed their presence, even if the water or food was dirty and offensive. Likewise, the most diligent search of twenty-four or more mosquitoes for malaria or yellow fever would in all probability fail to show a single malarial plasmodium or yellow fever bacillus. In the same way, hundreds of rat fleas might be caught and made to bite guinea-pigs or rats without the production of bubonic plague in a single instance. Do any of these negative observations disprove or discredit in the least degree our present views on the origin of the various diseases whose avenues of infection we have mentioned?
By what privilege then does our scientific friend, Warren W. Hilditch, of the Sheffield Laboratory of Bacteriology and Hygiene, Yale University, claim in The Popular Science Monthly of August, 1908, even the least knowledge of the transmission of disease by money from the bacteriological study of twenty-four bills, "the dirtiest I could obtain from various sources, such as railroad, trolley and theater ticket