But I bet that they could be carriers/vectors of many diseases. HIV being one. In that vein they could be like mosquitoes and transmit malaria, dengue fever, yellow fever, maybe even bubonic plague, and perhaps the more dangerous variant, pneumonic plague, Ebola virus, Hanta virus etc.
And Alan Moore's Swamp Thing back in the 80s theorized vampires being of an anaerobic nature, so that would add other diseases to the list, and just general virulent infections such as smallpox, diptheria, cholera, elephantiasis, staphloccocus Aureus, and other staph infections, and there is a whole host of blood borne pathogens that include hemorraghic (sic) fever. Some literally break the blood cells, which may make digestion easier.
Did you ever think that maybe vampires are perpetually constipated? Blood in the stolld make them very hard, and difficult to pass.