However, fortunately, here, as in other human affairs, imperfect as our knowledge is, it is sufficient to serve as a guide for practical conduct. Widely present as the pneumococcus is, we know well that it is powerless for harm except in unhealthful surroundings. There is another interesting feature of its life history which is of practical importance, and that is, like many other bacilli it is increased in virulence and infectiousness by passing through the body of a patient. Flushed with victory over a weakened subject, it acquires courage to attack a stronger. This is the reason why, in those comparatively infrequent instances in which pneumonia runs through a family, it is the strongest and most vigorous members of the family who are the last to be attacked. It also explains one of the paradoxes of this disease, that, while emphatically a disease of overcrowding and foul air, and attacking chiefly weakened individuals, it is a veritable scourge of camps, whether mining or military. When once three or four cases of pneumonia have occurred in a mining camp, even though this consist almost exclusively of vigorous men, most of them in the prime of life, it acquires a virulence like that of a pestilence, so that, while ordinarily not more than fifteen to twenty per cent of those attacked die, death-rates of forty, fifty, and even seventy per cent are by no means uncommon in mining camps. The fury and swiftness of this "miners' pneumonia" is equally incredible. Strong, vigorous men are taken with a chill while working in their sluicing ditches, are delirious before night, and die within forty-eight hours. So widely known are these facts, and so dreaded is the disease throughout the Far West and in mountain regions generally, that there is a widespread belief that pneumonia at high altitudes is particularly deadly. I had occasion to interest myself in this question some years ago, and by writing to colleagues practicing at high elevations and collecting reports from the literature, especially of the surgeons of army posts in mountain regions, was somewhat surprised to find that the mortality of all cases occurring above five thousand feet elevation was almost identical with that of a similar class of the population at sea-level. It is only when a sufficient number of cases occur in succession to raise the virulence of the pneumococcus in this curious manner that an epidemic with high fatality develops. That this increase in virulence in the organism does occur was clearly demonstrated by a bacteriologist friend of mine, who succeeded in securing some of the sputum from a fatal case in the famous Tonopah epidemic of some years ago, an epidemic so fatal that it was locally known as the "Black Death." Upon injecting cultures from this sputum into guinea-pigs, the latter died in one-quarter of the time that it usually took them to succumb to a similar dose of an ordinary culture of the pneumococcus.