Yet he was every bit a warrior, and he hunted death.

When he found it he confronted it, challenged it, tried to pin it in place like a lepidopterist pinning down a butterfly, so he could then dissect it piece by piece, analyze it, and find a way to confound it. He did so often enough that the risks he took became routine.

Still, death had never appeared to him as it did now, in mid- September 1918. Row after row of men confronted him in the hospital ward, many of them bloody and dying in some new and awful way.

He had been called here to solve a mystery that dumbfounded the clinicians. For Lewis was a scientist. Although a physician he had never practiced on a patient. Instead, a member of the very first generation of American medical scientists, he had spent his life in the laboratory. He had already built an extraordinary career, an international reputation, and he was still young enough to be seen as just coming into his prime.

A decade earlier, working with his mentor at the Rockefeller Institute in New York City, he had proved that a virus caused polio, a discovery still considered a landmark achievement in the history of virology. He had then developed a vaccine that protected monkeys from polio with nearly 100 percent effectiveness.

That and other successes had won him the position of founding head of the Henry Phipps Institute, a research institute associated with the University of Pennsylvania, and in 1917 he had been chosen for the great honor of giving the annual Harvey Lecture. It seemed only the first of many honors that would come his way. Today, the children of two prominent scientists who knew him then and who crossed paths with many Nobel laureates say their fathers each told them that Lewis was the smartest man they had ever met.

The clinicians now looked to him to explain the violent symptoms these sailors presented. The blood that covered so many of them did not come from wounds, at least not from steel or explosives that had torn away limbs. Most of the blood had come from nosebleeds. A few sailors had coughed the blood up. Others had bled from their ears. Some coughed so hard that autopsies would later show they had torn apart abdominal muscles and rib cartilage. And many of the men writhed in agony or delirium; nearly all those able to communicate complained of headache, as if someone were hammering a wedge into their skulls just behind the eyes, and body aches so intense they felt like bones breaking. A few were vomiting. Finally the skin of some of the sailors had turned unusual colors; some showed just a tinge of blue around their lips or fingertips, but a few looked so dark one could not tell easily if they were Caucasian or Negro. They looked almost black.

Only once had Lewis seen a disease that in any way resembled this. Two months earlier, members of the crew of a British ship had been taken by ambulance from a sealed dock to another Philadelphia hospital and placed in isolation. There many of that crew had died. At autopsy their lungs had resembled those of men who had died from poison gas or pneumonic plague, a more virulent form of bubonic plague.

Whatever those crewmen had had, it had not spread. No one else had gotten sick.

But the men in the wards now not only puzzled Lewis. They had to have chilled him with fear also, fear both for himself and for what this disease could do. For whatever was attacking these...