It would seem like the likeliest scenario for a future dark age is neighboring towns (or tribes, I suppose) tell tales about how everyone who goes into area X eventually dies not long after.

Every few generations some brave, ballsy kid shrugs off these old wives tales, goes and camps out in area X, and gets ill and dies not long after. The legends endure... at some cost.

Of course, it depends how strong the radioactivity/toxicity is. If it leads to cancer 30 years down the road, and no immediate signs, then it gets a lot messier.