Well, back home, the public school had severe mold issues, they actually closed it down for a few months until it was taken care of.
YES, it is dangerous, molds have ways of weakening your immune system and you can ill afford that since you are being treated for cancer.
I re-read your post, since you are leasing, I would certainly let the landlord know, if he does not do anything, then a move may be in your future.
its unsafe i would let the owner of the place fix the mold prob and if he dont i would call a helth inspectre and move out...
You stated you are leasing! Call the landlord and state your problem!!
Mold is never a good thing to have in your house. Spray it with bleach water, as bleach kills mold. Also, change the filter in your furnace/air conditioner frequently with filters that capture mold. I would also suggest investing in an air purifier. Hope this helps - Blu