What he found was that a yellow-fever patient could be visited without hazard so long as the visit was made within ten to twelve days after the patient became ill. But beyond that period, even after the patient had died and the body had been removed from the house, family or visitors were in mortal danger. Hence the sick man could not possibly be the source of contamination. Hence there had to be a period of “extrinsic incubation,” as Carter named it.
