You can see in the Mishna Seder Purity that there is a certain kind of uncleanliness that emanates from a dead body. This is called father of fathers of uncleanliness. In fact you can see in the Book of Numbers that this is the most severe type of uncleanliness that there is. All other types are fathers of uncleanliness or the derivates.
[I spent a lot of time on this while at Shar Yashuv and the Mir in NY but I have forgotten most of it.] But even though I have forgotten most of this subject, it does occur to me to mention that there really is no reason to go to graves of the righteous.
In Torah, one is not supposed to intend to be connected with dead people. This is well explained in the Nefesh HaChaim of Rav Chaim of Voloshin [a major disciple of the Gra.]] Rather, one is supposed to intend to be connected with God. To intend to be connected with the dead is an extremely disgusting sort of idolatry.
