Maybe they weren’t really in the apartment with him.
Well, they were there afterwards because the media was there filming it 24/7. But either they did greatly limit their contact with him from the get-go, revealing they knew more than they admit, OR it is not as easy to get in those early/mid stages. Or they were extremely lucky. I don't really go with the lucky theory.
That’s always been my theory, they knew in advance Duncan was exposed to Ebola(He probably told them before he left from Liberia) and they left the apt, they were never in that apartment to begin with