The first is terrorism. We haven't dealt with it. It's an annoyance. Clearly, to the government, 9/11 was not a wake up call to root this out and destroy it. We're managing it. Not going too far, might upset the oil cartel, not too concerned about another strike, and by the way, a wonderful opportunity to just up the surveillance on everyone, so the big win is to keep some terrorism going.
The second is ebola. Something in the jungles of Africa is a reservoir. Every once in a while, it get out. This time it is in the cities and it seems to be persistent. Bodies are being left in the streets. It is transmitted in blood, urine, saliva, and feces. Corpses remain infectious for several days. Infected people will not show symptoms for two to ten days after infection.
Put those two problems together.
Make it deliberate, say a committed group of terrorists that wanted to do the maximum damage to the United States.
Deliberate infection of a number of people in Africa that then immediately fly to several U.S. cities. They would not have to know they are infected, but they might. Either the individuals or their handlers spread it as much as possible. Give a spray bottle to a woman and let her walk through a mall, stopping and cooing over children in strollers. The day care centers would do the rest. Give a bottle of fluid to a hospital worker in the kitchen that is delivering food and utensils to rooms. I'm sure you'll think of others.
Let's include Mexico City and have it delivered to the areas where the illegal immigrants are gathering to make the crossing. Since our own government is quietly dispersing thousands of people in cities all over the country, all those destinations become infection sites.
In the end, it might not kill that many people. It's the disruption, the cost, the travel bans, and the terror. Because we didn't believe it could happen. We didn't recognize the threat. We didn't believe people could do that. We thought we could contain terrorism.
"No one could have imagined them taking a plane, slamming it into the Pentagon" --Condoleezza Rice