-too high interior humidity
-not enough (if any) wall insulation
Only by lowering the humidity and by insulating the walls will you solve the problem.
Is it the whole house?
You are probably going to have to remove the drywall where you have the problem. The insulation might have mold damage also. The wall studs probaly have mold that need to be treaded with a solution of bleach and water. You might want to check plumbing in that area to make sure you do not have a leak.
Do not do any work yourself without a mask.