Thomas Eric Duncan has the distinction of being America's Patient Zero - the first but not the last person to develop Ebola symptoms in the United States.
Is he a US citizen? No, he's Liberian.
Is he a resident of the United States? No, he landed at Washington's Dulles Airport on September 20th, in order to visit his sister and having quit his job in Monrovia a few weeks earlier.
So he's a single unemployed man with relatives in the US and no compelling reason to return to his native land. That alone is supposed to be cause for immigration scrutiny. http://www.steynonline.com/6586/ebola-yes-bagpipes-no
And the disease is expected to reach Britain and France in three weeks: http://www.voanews.com/content/ebola-outbreak-spread-to-france-britain-late-october/2473054.html