This bothers me too. The only way it makes sense to me is that because Duncan was in the early stages of the disease, it(ebola) was not as active and had not multiplied/replicated at that time.
It still remains to be seen, but this looks to be a distinct possibility. What are the odds that Louise AND all three of the "close" contacts just happen to have an unexplained immunity? No, it seems that in the immediate onset of symptoms, it is not as contagious as when the patient is secreting copious fluids.
That said, I'm stunned Louis continues to be asymptomatic. She still has some time to go, but how she wasn't patient number 2 is beyond me.