ATLANTA — The first Ebola victim to be brought to the United States from Africa was safely escorted into a specialized isolation unit Saturday at one of the nation’s best hospitals, where doctors said they are confident the deadly virus won’t escape.Fear that the outbreak killing more than 700 people in Africa could spread in the U.S. has generated considerable anxiety among some Americans. But infectious disease experts said the public faces zero risk as Emory University Hospital treats a critically ill missionary doctor and a charity worker who were infected in Liberia. The U.S. Centers for Disease Control and Prevention has received “nasty emails” and at least 100 calls from people saying, “How dare you bring Ebola into the country!?” CDC Director Tom Frieden said Saturday. “I hope that our understandable fear of the unfamiliar does not trump our compassion when ill Americans return to the U.S. for care,” Frieden said. Dr. Kent Brantly and Nancy Writebol, who will arrive in several days, will be treated in Emory’s isolation unit for infectious diseases, created 12 years ago to handle doctors who get sick at the CDC, just up the hill. It is one of about four in the country equipped with everything necessary to test and treat people exposed to very dangerous viruses.