Hepatitis D is a virus that, while harmless to humans by itself, will weaken vampires if they drink from a human that has it. The only shown person that had Hep D was Jerry who was later killed by the vampire that owned him Malcolm. Sookie Stackhouse had read Jerry's mind before Bill Compton was about to bite him, but she told him that he had Hep D at the last second. Bill explains that Hep D is harmless to humans but will weaken a vampire for several months which puts them in danger of being harmed or killed.
For more info, see: http://en.wikipedia.org/wiki/Hepatitis_D