viruses like Ebola are notoriously sloppy in replicating, meaning the virus entering one person may be genetically different from the virus entering the next. The current Ebola viruss hyper-evolution is unprecedented; there has been more human-to-human transmission in the past four months than most likely occurred in the last 500 to 1,000 years. Each new infection represents trillions of throws of the genetic dice.
No offense taken. Natural Selection favors the more effective virus. Frankly, the longer time frame offers some explanation for why Ebola is not burning itself out.