But the case is also noteworthy for another, potentially positive reason: Nearly 50 people were exposed to Ebola before the nurse, and none of them has been diagnosed with the disease.

This group of neighbors, family members and first responders are being watched carefully by health authorities. They had some degree of close contact with Duncan during the four-day period when he was contagious – from when he started showing Ebola symptoms on Sept. 24 to when the hospital finally admitted him on Sept. 28. They didn’t take any Ebola-specific precautions. They didn’t know he was infected. Some stayed in the same apartment as Duncan as his condition worsened. Yet, so far, they have not gotten sick. And their 21-day Ebola incubation period started before Pham’s.

“That the casually exposed are not getting sick, it’s reassuring,” said Dr. Julie Fischer, an associate research professor of health policy at George Washington University. “What we’ve seen so far, it’s not surprising and it’s not shocking.”