Wow.
Ebola 'first' came to the U.S. back in 1989 when RestonVirus was discovered in a group of crab eating macaques imported from the Philippines.
Exposure to Reston by workers of the quarantine zone was confirmed, but none ever showed signs of infection, which is evidence that Reston has substantial difficulty replicating in humans.
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Although the first person to be diagnosed with Ebola in America was Thomas Duncan - a man who helped a pregnant woman who died of Ebola before flying here and lying about having contact with someone who had Ebola.
Other healthcare workers have been flown in under full quarantine conditions to specialized hospitals in order to be treated for the virus - but this is the first time someone has been diagnosed with the virus while in the U.S.
One of the nurses who cared for him tests positive - marking the first time Ebola has been transmitted within the U.S.
Bluntly - it has come here (and is likely still coming here) because of the hubris of politicians who believe they can write legislation to end a pandemic or give a speech to cure all illnesses (or something). They have not taken proper action regarding the restriction of travel and the establishment of monitored flights for support/relief.
Now... why is it spreading despite how "hard it is to get?"
Because, again, Politicians believe they are in control of things and don't understand that a virus doesn't play politics. We are food, and so long as food continues to play with the microscopic predator that consumes it - the predator will take any opportunity it has to feed.
That's just how it goes.
These patients are going to begin cropping up in places where the staff are not nearly as well trained or equipped to deal with Ebola - and the staff are going to contract it. It's how epidemics work.