I am not defending the ER.
However, in just about every outbreak since 1976 the first patients were mis-diagnosed.
The initial symptoms are similar to malaria and a severe case of the flu. This year flu and other viruses are presenting much earlier than usual.
There is no immediate test of Ebola. The diagnosis takes a couple of days to perform.
THIS is one of the reasons the first victims are usually the medical staff and first responders.
So put a fence around Dallas for 3 weeks. Of course it would have been simpler and cheaper to quarantine all passengers from certain areas first. But this is your guberment at its finest.