How is it possible that its spread did not change significantly in the last couple of decades?

Im wondering how can very uncommon diseases remain in a relatively stable equilibrium. For example Show more Im wondering how can very uncommon diseases remain in a relatively stable equilibrium. For example if it started to spread the spread could accelerate and become an epidemic. However if it doesnt spread why doesnt it wither away? As I understand the ebola for example is highly contagious nearly always fatal and kills quickly. So quickly hat the target has not much time to pass it on to others. If it spread just a little better it would become an epidemic just a little slower and it would disappear. How is it possible that its spread did not change significantly in the last couple of decades? It kills a few dozen maybe a few hundred people per year these numbers remain largely unchanged.