Just throwing this out there.

Does anyone remember the Jake Gyllenhall movie "Jarhead"? The solders were given untested drugs, just in case Saddam used sarin gas or something. What if that had something to do with Saul's not getting sick @ ground zero. So that would make Michael immune too. Not so sure about Angel, because he has never been in Iraq or Afghanistan. Of course the simplest theory is that the antibiotics are making him immune.