For public health workers screening more than 1,000 air travelers who arrive each week in the United States from Ebola-stricken West Africa, one symptom above all others is supposed to signal danger: fever. So long as an individual's temperature does not exceed 101.5 degrees and there are no visible symptoms of Ebola, health authorities say it should be assumed the person is not infectious. Yet the largest study of the current outbreak found that in nearly 13% of "confirmed and probable" cases in Liberia, Sierra Leone, Guinea and elsewhere, those infected did not have fevers. The study, sponsored by the...