It does strike me as curious that the nurses were infected but no family members or other bystanders were. The family, the bystanders, and the nurses all would have been exposed to vomit, saliva, and air droplets, but only the nurses would have been exposed to with blood and waste. This suggests to me that the virus is in infectious concentrations the blood and the gut. In environments like the U.S., therefore, the disease might not be expected to spread as easily as in West Africa, where the sanitations standards are different.
Studies have suggested that the viral load by type of body fluid is quite idiosyncratic -- one patient may shed a lot of virus through saliva, another patient very little or none, etc.
So everyone should be careful extrapolating from one patient to others.