halachik source of negiah

Question

quick question rabbi -- What is the technical halachik issur of Negiah? I have heard that it is b/c of an issue related to Nidah...Could you possibly give me a place to look or some sources so I can study a bit about it? (not looking for heter! -- I was just curious what the origins and development of the halacha is...) Its the type of halacha that makes sense to us, and we kinda assume it to be, but what the technical reasons behind it i'm not sure about.... thanks again rabbi...