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.
In addition, visitors from Liberia have the fifth highest "visa overstay rate" in the United States. That's to say, they understand very clearly that all that matters is getting in. Once you're in, they'll never get you out.
And, of course, Liberia is one of the hottest spots of Ebola's West African "hot zone". It's been all over the front pages, except apparently in The US Customs & Border Protection Staff Newsletter, where it rated a solitary "News In Brief" item at the foot of page 37.