For a long time nobody knew just how to combat these pestilences, and nowhere were rational measures adopted aiming to prevent them from spreading. We have seen the bitter truth of this statement in connexion with the endless Napoleonic wars. One reason for the neglect of preventive measures was the belief that these pestilences broke out spontaneously when large numbers were crowded together under miserable living conditions. The physicians of that time, in their efforts to explain the sudden appearance of these pestilences, arrived at the conclusion that they were autochthonic. But modern medical science, realizing its limitations, contents itself with the hypothesis that the original cause of these diseases is not to be ascertained, and with the knowledge that the infective agents in the case of almost all of them have been discovered, and that an outbreak of any infectious disease in any specific locality signifies that the germ of that disease must in some way have been deposited there. It was precisely the belief in the spontaneous origination of pestilences that led people to neglect watching for and isolating, with all possible dispatch, the first cases—a measure which is to-day looked upon as the most important means of preventing the dissemination of a disease. The belief in the autochthonic origin of diseases continued to prevail until the first half of the nineteenth century. Hecker upheld it in a discussion of the plague-epidemic that occurred during the Russo-Turkish War of 1769–70; he believed that the intermittent fever prevalent in the Danube countries passed over into putrid fever, with or without petechiae, that carbuncles and buboes gradually developed, and that putrid fever was thus converted into bubonic plague. ‘It is therefore in all probability true’, says Hecker, ‘that the outbreak of plague in the Russian army in the year 1770, as well as in the year 1828, was not caused by direct infection from the Turkish troops, but was merely an independent development from intermittent fever and spotted fever.’