John Tyndall, in combating Bastian’s views showed that boiled infusions left open to the air in a closed box through which air circulated did not show any growth of organisms provided the air was so free of particles that the path of a ray of light sent through it from side to side could not be seen (Fig. 7). Or if such sterilized infusions were exposed to dust-free air, as in the high Alps, the majority showed no growth, while all infusions in dusty air did show an abundance of organisms. Tyndall’s experiments confirmed those of Pasteur and his predecessors and showed that the organisms developed from “germs” present in the air falling into the liquids and not spontaneously. While Tyndall’s experiments were of great value as indicated, they probably were harmful in another way. These “germs in the air” were considered by bacteriologists as well as laymen to include necessarily many _disease germs_ and to indicate the very general, if not universal, presence of these latter _in the air_. This idea led to many erroneous practices in sanitation and disinfection which even to this day are not eliminated. CAUSATION OF DISEASE. The transmission of disease from person to person was recognized by the ancients of European and Asiatic countries. Inoculation of smallpox was practiced in China and India probably several thousand years ago and was introduced by Lady Mary Wortley Montague into England in 1721, from Constantinople. These beliefs and practices do not seem to have been associated with any speculations or theories as to the cause of the disease. Apparently the first writer on this subject was Varo, about B.C. 70, who suggested that fevers in swampy places were due to invisible organisms. The treatment of wounds during the thirteenth and fourteenth centuries by hot wine fomentations and by the application of plasters was based on the theory that the _air_ brought about conditions in the wounds which led to suppuration. These practices were indeed primitive antisepsis, yet were not based on a _germ theory_ of the conditions which were partially prevented. Fracastorius (1484-1553), in a work published in 1546, elaborated a theory of “disease germs” and “direct and indirect contagion” very similar to modern views, though based on no direct pathological knowledge. Nevertheless Kircher (mentioned already) is usually given undeserved credit for the “contagium vivum” theory. In 1657 by the use of simple lenses he observed “worms” in decaying substances, in blood and in the pus from bubonic plague patients (probably rouleaux of corpuscles in the blood, certainly not bacteria in any case). Based on these observations and possibly also on reading the work of Fracastorius, his theory of a “living cause” for various diseases was published in 1671, but received little support.