OK, so it's unlikely to be a cavity problem. Could it be a minor leak, say from a joint or radiator ? could it be raining in, you don't say if it is on an outside wall or not ?
Is the bathroom to bedroom wall a cavity wall or not ?
Alternatively, if this is condensation. Condensation will go to the coldest place it can, would you say that the bathroom and the wall in question is the coldest place ?
Not easy without seeing it, remember that mildew is a fungus like a plant it will grow back until not only the problem is cured but also the mildew is gotten rid of. This might even mean re-plastering to be sure. But don't do this unless necessary and until you are sure the cause is fixed.
<DIV><FONT color=#0000ff size=2>What is the best way of getting rid of