It's not what you might think. Ezra Klein at vox.com explains.
"Endemic" is, for now, the scariest word in the Ebola epidemic. It would mean that Ebola becomes a constant threat rather than an episodic one. It would mean that every health system everywhere in the world would have to constantly worry about an outbreak of Ebola. As Thomas Frieden, director of the CDC, told me, if Ebola became endemic, "anytime anyone comes to our country from a place where they've had Ebola, we'd have to see if they had a fever, and if they did, we would have to treat them as if they did have Ebola until we ruled it out."
And if Ebola becomes endemic in West Africa, all this will get much worse. In addition to the ongoing breakdown in basic services, it will be harder for West Africans to travel because few countries will let them in, it will be harder for them to trade because fewer businessmen will want to travel to the region, it will be harder for them to invest because international bankers will be scared off by the disease. It could set the region's development back decades. That's worth panicking over, not because it might kill vast numbers of Americans, but because it might cause a vast amount of human suffering.
And that is why it is worth the effort and gobs of money to control Ebola yesterday.