I read one reason may be that Ebola is not an airborne virus, unlike the common cold, say. In other words, what they seemed to be saying was that airborne viruses tend to be transmissible during incubation, while non-airborne viruses generally aren't. Now, why that should be the case, at the microbiological level, I have no idea... I'm not an expert. It's just something I read.
droplets from a person’s saliva expelled during a sneeze are still dangerous though, right?