The scientific discoveries of the U.S. Army Yellow Fever Board of 1900 are well known as are the Army physicians who led the board. Walter Reed, of course, is the best known, but James Carroll, Aristides Agramonte, and Jesse Lazear are also known, if not nationally, to their local communities. This article deals not with the known but with the unknown, meaning the volunteers who subjected themselves to the ravages of yellow fever and the real possibility of death. The year 1900 was known as a "yellow fever year" among the locals in Cuba because in the preceding year the epidemics had been relatively mild. Beginning its work in June 1900 in the midst of a deadly epidemic, the board conducted a truly remarkable set of experiments that set a benchmark for controlled clinical trials and informed consent. Because no animal model was known to be susceptible to yellow fever, they used human volunteers for their experiments. These volunteers were recruited from among Spanish immigrants and were accepted from soldiers and two civilians who volunteered. Over 30 men participated in the experiments, and 22 developed yellow fever. With expected death rates of 20% to 40%, it is incredible that none of these volunteers died. In 1929, the U.S. government honored the Americans who volunteered by placing their names on a Roll of Honor published annually in the Army Register. The successes of the 1900 U.S. Army Yellow Fever Board were truly remarkable, and many of the successes were made possible by the men who volunteered, some repeatedly, to risk their lives "in the interest of humanity and the cause of science."