There was panic in Habana, and mutterings of the mob--and who can blame that mob, for human life is sacred. But there was Assistant-Surgeon James Carroll, unsentimental as an embalmer and before all else a soldier,--he had just then come back to Habana to settle certain little academic questions. “We can wipe out yellow fever now, we have proved just how it gets from man to man--_but what is it causes yellow fever_?” This is what Reed and Carroll asked each other, and everybody must admit that it was a purely academic question, and I ask you: was it worth a human life (even of a Spanish immigrant) to find the answer? Myself I cannot answer yes or no. But Reed and Carroll answered yes! Starting out as soldiers obeying orders, as humanitarians risking their hides to save the lives of men, they had been bitten by the virus of the search for truth, cold truth--they were enchanted with the glory that comes from the discovery of unknown things.... They were sure there was no visible bacillus, nor any kind of microbe that could be seen through the strongest microscope to cause it--they had looked in the livers of men and the lights of mosquitoes for such a germ, in vain. But there were other possibilities--magical possibilities, of a new kind of germ that might be the cause of yellow fever, an ultra-microbe, too immensely small for the strongest lens to uncover, revealing its existence only by the murdering of men with its unseen mysterious poison. That might be the nature of the germ of yellow fever. Old Friedrich Loeffler--he of the mustaches--had found such little life making calves sick with foot-and-mouth disease. And now if Reed and Carroll could show the microbe of yellow fever belonged to this sub-microscopic world too! Walter Reed was busy, so he sent James Carroll to Habana to see, and here you find James Carroll, intensely annoyed because those experimental cases of Guitéras had died. Guitéras--do you blame him?--was in a funk. No, Carroll mightn’t draw blood from yellow fever patients. Indeed not, Carroll mightn’t even bite them with mosquitoes. What was most silly, Dr. Guitéras would rather not have Dr. Carroll make post-mortems on the dead cases--it might enrage the population of Habana. “You can imagine my disappointment!” wrote Carroll to Walter Reed, with indignant remarks about the frivolous fears of ignorant populations. But did those deaths stop him? Not Carroll! By some unexplained sorceries he got hold of some good poisonous yellow fever blood, and filtered it through a porcelain filter that was so fine no visible microbe could get through it. The stuff that came through that filter Carroll shot under the skin of three non-immunes (history doesn’t tell how he induced them to stand for it)--and presto! two of them got yellow fever. Hurrah! Yellow fever was like foot-and-mouth disease then. Its cause was a germ maybe too little to see, a microbe that could sneak through fine-grained porcelain.[1]