Walls and Ceilings - mold on wall when removed wallpaper

I just removed wallpaper from the bathroom walls. On 2 of the walls, there is mold/mildew/? on the wall. It is black circles and areas.

1. What do I do to prepare the wall for painting or wallpaper? Is Kilz enough, or do I need to remove the drywall and replace it, to remove the mold completely and stop it from spreading? How do I know if the mold is a health hazard?

2. These 2 walls surround the chimney that serves the furnace in the basement. I assume the moisture is from a leak from the top of the chimney. I just had a new crown put on it since it was deteriorating, and some tuckpointing done. Hopefully that will fix the problem (if it wasn't fixed years ago...it's a 40 year old house.) But could the moisture be from condensation or something involving the chimney, and not from a leak?

Thank you!

Both moisture and heat are required for mold/mildew to grow. Hopefully you have fixed the moisture issue. Before priming with kilz you should wash the area down with some bleach water. Hopefully this will both kill and remove the mold/mildew. When dry, priming with kilz will help to both cover any stains and make it harder for any mildew not killed to grow.

could always play it safe and remove old drywall and replace. The cost isn't much compaired to the peice of mind knowing your mold free. :)

As long as you're already painting, I would go the extra step and replace the drywall as well. Not much extra work for peace of mind.