ATLANTA — An American doctor infected with the Ebola virus in Africa arrived in Atlanta for treatment Saturday, landing at a military base, then being whisked away to one of the most sophisticated hospital isolation units in the country, officials say. It marks the first time anyone infected with incurable Ebola, considered one of the world's deadliest diseases, has been brought into the country. A second American aid worker infected with the virus was expected to arrive within a couple days.