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mark that it was so hard to keep different races of germs from getting mixed up that Koch would have to have a separate laboratory for each species of microbe. . . . In short, Virchow was very sniffish and cold to Koch, for he had come to that time of life when ageing men believe that everything is known and there is nothing more to be found out. Koch went away a bit depressed, but not one jot was he discouraged. Instead of arguing and writing papers and making speeches against Virchow he launched himself into the most exciting and superb of all his microbe huntings—he set out to spy upon and discover the most vicious of microbes, that mysterious marauder which each year killed one man, woman, and child out of every seven that died, in Europe, in America. Koch rolled up his sleeves and wiped his gold-rimmed glasses and set out to hunt down the microbe of tuberculosis.

Compared to this sly murderer the bacillus of anthrax had been reasonably easy to discover—it was a large bug as microbes go, and the bodies of sick animals were literally alive with anthrax germs when the beasts were about to die. But this tubercle germ—if indeed there was such a creature—was a different matter. Many searchers were looking in vain for it. Leeuwenhoek, with his sharpest of all eyes, would never have found it even if he had looked at a hundred sick lungs; Spallanzani's microscopes would not have been good enough to have revealed this sly microbe; Pasteur, searcher that he was, had neither the precise methods of searching, nor, perhaps, the patience, to lay bare this assassin.

All that was known about tuberculosis was that it must be caused by some kind of microbe, since it could be transmitted from sick men to healthy animals. An old Frenchman, Villemin, had pioneered in this work, and Cohnheim, the brilliant professor of Breslau, had found that he could give tuberculosis to rabbits—by putting a bit of the consumptive's sick lung