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130 for the season, relieved of his work as professor of the Army Medical School which had been suspended for the period of the Spanish War and was not to resume its sessions until October 1901. Meanwhile, Reed, as he wrote to Carroll, who was still in Havana, was "tied down to the Army Examining Board." 46

The Yellow Fever Commission had succeeded in demonstrating to the satisfaction of the world the method of transmission of the disease, but the discovery of the activating cause of the disease itself was unfinished business. In 1898, Friederich A. J. Löffler and Paul Frosch had demonstrated that hoof-and-mouth disease in animals is due to something called, for want of a better name, a "virus." Prof. William H. Welch, who had taught Reed at Johns Hopkins, and who had been a fellow-pupil of Loffler's under the great Robert Koch, called Reed's attention to the Löffler-Frosch findings in the early summer of 1901.

To Reed and Carroll, it appeared possible that the same sort of substance might be the cause of yellow fever. A brief outbreak of tht disease in Santiago de las Vegas offered an opportunity to put the supposition to the test, so Carroll was sent back to Cuba in August 1901 to carry on the experiments. The tests showed that the infective agent was present in the blood and in the blood serum of fever patients, and that the power to produce fever persisted even after the serum had passed through "the pores of a filter which ordinarily serves to prevent the passage of all known bacteria." 47 It was to be yet another quarter of a century before it was finally established that the infecting agent of yellow fever is not a visible "parasite" such as Reed and Carroll sought, but is a virus which filters had not yet trapped nor microscopes revealed.

In the little more than a year which remained of what has been described as the "fifty-one years of Walter Reed's industrious, blameless life," 48 Reed lost no appropriate opportunity to give the medical world the lessons learned in the work of the Yellow Fever Commission. Besides his appearance before die Medical and Chirurgical Faculty of Maryland at Baltimore in April 1901, and his second appearance before the American Public Health Association in September, both of which have already been referred to, he appeared before the