In a country where a surgeon can leave a scalpel, scissors of clamps inside a patient, a country where a sizeable portion of NHS staff are barely speaking English or are young and reckless, is it so hard to believe that a mistake can be made by someone in the handling of infected bedsheets or such?
Let's not forget that viruses can mutate.
As I've said before, this might not be the one that gets us, but at some point you will say 'we've been here before, calm down, it's a non-story' and 90% of everyone dies.
Edit: Just because we CAN bring someone back, doesn't make it a good idea. That goes for the past as well as the now. If it really is such a 'serious threat', why don't we just send a big ol' C17 with a load of Hazmat gear, set up a base IN AFRICA with the qualified personnel and use that as an aid/ research station. The cynic in me says this obviously hasn't been thought of.