Santa Claus is the immortal man behind the spirit of Christmas. Santa Claus's predicament, being attacked and bitten by a Vampire, is the central plot device of the 'Grim Adventures of Billy and Mandy' TV Christmas special, Billy and Mandy Save Christmas.
Back History
According to Grim, he and Santa attended College together, and as alluded to in the special, continued to stay in touch years afterwards as his wife, Nancy Claus, also knows Grim personally. According to Nancy, Santa had always had, in her own words, a "odd desire to give presents to complete strangers," and had shown her the joys of giving, (blood,) not just recieving.
After the two of them got married, they perchased a "Pole of their own". Knowing that that making Christmas every year would be a lot of work, they both agreed to divide the work equaly amongst the two of them. Over the years, however, Santa became more pre-occupied with checking his 'naughty and nice' list and updating it, leaving his wife to pick up his slack. In Nancy's own words, Santa "works one night out of the year," while she is "working her fingers to the bone the other 364".
Santa explains to Mandy at the end of the Christmas special that it was actually the 6th or 7th time Nancy turned him into a vampire, but he always manages to return back to his "jolly old" self.