(CNN) -- How did this happen? That is what many people were asking Wednesday, after news broke that a health care worker had flown commercially at a time she was supposedly monitoring herself for a possible Ebola infection. Amber Vinson, 29, is the third person to be diagnosed with Ebola in the United States -- after Thomas Eric Duncan, the Liberian national who fell ill after coming to Texas, and Nina Pham, who like Vinson had cared for Duncan at Dallas' Texas Health Presbyterian Hospital. As with Pham, many questions surround how Vinson contracted Ebola despite efforts to prevent health care workers like them from getting the disease. Now they are compounded with others about safeguards for those who had contact with Duncan, who succumbed to the disease on October 8, to prevent the disease from spreading further.
NEW: CDC official: Those being monitored for Ebola may be put on "no-board" list . Vinson flew commercially in the time she was supposed to be self-monitoring for Ebola . Dallas health care worker called CDC about temperature, wasn't told not to fly, official says . CDC chief says that Vinson shouldn't have flown commercially, used "public transport"