This is more of a question than a theory.
Can anyone think of a character that Jacob has touched that has died up until the point in which Jacob died? I think maybe Jacob's touch is what protected them from the smoke monster and dying on the island. Now that Jacob is dead they may no longer be protected. The Others may not be protected either along with Richard. Let me know if you can think of anyone. I know we saw Locke die but that was off the island and possibly away from Jacob's protection AND it was not the SM that killed him. If this is the case then anyone but the smoke monster can kill Jacob. There is no wonder why he kept hidden away from everybody and why he needed bodyguards.