Even the foreign aid medical infrastructure is inadequate, not enough resources and too overworked to maintain proper isolation protocol, hence all the foreign aid workers getting infected.
I strongly suspect that a lot more people survive ebola than we hear about because they stay hidden at home with only the hardedst hit cases risk going to the hospital.
So it is more like "Year after year, you have some people get sick, some die, some live. One day somebody gets sick and dies fast and painfully. A few weeks later the rest of their tribe/family is dead or dying. A few weeks after that, the space aliens start coming in, but people continue to die."
Think about how many folk in Europe or the US are against vaccination, or think that big pharma is trying to wipe out everyone.
Now consider how many people are of those opinions in areas that have historically been governed by unbelievably brutal European empires that treated people living there like livestock.