I've heard of burial sites and archeological digs coming across radioactive items and bodies, and surely ancient peoples would have had encountered examples of uranium on the surface. Did any of the ancient peoples have a concept of radiation poisoning and it's cause?
Sort of. People noticed miners working in Joachimsthal died of an unknown lung ailment at an alarming rate in the 1400s. These mines were likely filled with huge quantities of Rn (which causes lung cancer), though they obviously didn't know about radioactivity. http://www-ns.iaea.org/tech-areas/communication-networks/orpnet/documents/cn223/8-chambers-keynote.pdf