The thing with the Ebola virus is, it usually kills the host or dies off before it can mutate (mortality rates vary between 50% and 90% - it did hit the magic 100% once, in Russia, but then it only infected one person). That's unlike 'flu viruses and most bacteria, which last and mutate because they're generally long-lived and (these days) non-fatal. The other thing with 'flu, etc, is that people take treatments but don't always complete the course, or don't renew a prescription (or continue taking the remedy) once they feel like they're over the worst. That way, enough of the pathogen survives to develop a resistance. Over time, it becomes immune to the treatments. It's not stronger, because there's always a trade. If anything, it's weaker. It's just that it's more of a bugger to shake should you get it.
The Ebola variant in the West African cases is already the worst one, what used to be called Ebola Zaire. As a rule of thumb, if a disease mutates into an airborne form (which takes time and a need plus opportunity on the part of the disease to do so, usually as a response to effective treatments like vaccines - and in the case of Ebola, there are none, and simply keeping the patient hydrated is not enough to 'force' a mutation), that new form will be less, not more, virulent. Since it is airborne, it will naturally make more people sick and more may even die, but the disease itself won't be as effective (the mortality rate may even halve).
In any case, Ebola Zaire hasn't mutated to become airborne in any of the previous outbreaks, of which there have been several, and Reston probably came about as a result of an escaped experiment (but was probably a distinct variant of the virus in the first place), so that could well mean that it can't, for whatever reason.
