"I don't recall the last time, if ever, a dangerous pathogen escaped from CDC."

There have been a whole series of incidents, suspected incidents and the most recent one was just in July. A major screw up involving 86 people at the CDC.

Anthrax at the CDC: 86 workers possibly exposed - CNN.com


That said, between the CDC and another gov lab that specializes in nasty bugs you can bet that every deadly virus known to man is kept and studied in these labs and the use of them as bio agents is studied for attack and defense. The same holds true for most major powers.

The Ebola patients being brought into the US today aren't a threat given proper handling.