I wouldn't place too much value on the "foreign contamination" theory. You may be right, but all mold dies in the dry. All mold lives in the wet. It grew because it could.
My advice is to move. You obviously live in a really embarrassing place like Florida or something because you a) don't have a basement and b) have conveniently talked around your geographic location. If you can't move, ask a Brit here, odds are good that one of them must have a "damp" problem.
Here in Michigan it's mad humid in the summer. However it's also SUPER dry in the Winter so maybe that evens it out? We also have basements so...I've had mold there, no question. What I've learned though is that you will NEVER defeat mold without drying the place out first. Once it's dry, then you can get with the bleach and whatnot, but before that it's a waste of time.