Leak in your bathroom, be very careful when repairing it, I finally got my apartment managers to fix my bathroom after 5 years of flooding my apartment and the hallway outside my suite, and they encounter loads of black mold, the toxic stuff.
here are some pics of the disaster my bathroom is in
They have sealed off my bathroom till the chemicals kill the mold, then they have to replace all the supports for the wall, all the 2x4's and 4x6's, the gyprock and insulation. Once they are done the bathroom, they are doing the outside hallway. Heres a link for some information on it.
Toxic Mold