They could incubate zombies in the target country unnoticed. Walking in a mall and bump against someone scratching the skin with a pin containing microscopic elements of virus. A crowded bar on Saturday night. People constantly bumping in to each other. Random attacks on the street. You name it and it can be done. It's the same as any other hysteria based terrorist attack. You make a couple people in one area sick and it gets all kinds of publicity. Then the nation is afraid and wants all kinds of knee jerk actions taken. Most of these things aren't at all serious. Only one needs to be and it can be devastating.