Ebola survivor relapse

Ebola research enters completely new territory with UK nurse previously infected and cleared, relapsing and in critical condition. News of this is troubling with respect to the survivors of the 2014/15 outbreak. Interesting however is how this relapse fits in to the bigger picture of previous outbreaks where relapses have not been documented to have occurred - once cleared, individuals seems to stay clear of the virus. That begs the obvious question as to what's different about this UK case?