Deena, you're a mind-reader, because that was the next thing on my list. I'd been looking at something that would a) be viral or possibly bacterial in nature (remembering that one of my early theses was on the Black Death), b) not evince symptoms in the initial carrier, but c) trigger a chromosomal malfunction in said carrier's descendents.
The first thing that comes to mind is haemophilia (I can never remember whether it's factor 7 or factor 8 that's affected there), because the results were so damned visible, courtesy of the Tsarevitch and Rasputin. And most of the conventional wisdom when I was growing up was that it was through Victoria's line, her father was what, 57, when she was conceived, a mutation in the sperm.
But of course there are other conditions as well.
My neurologist is actually in a pool of highly research-oriented doctors who believe that MS is triggered virally.
Since the science in this one needs to be accurate - it's the lynchpin for the story, and the main reason the two older vampires have to work with their potential dinner, instead of eating it - research is indicated.
The grandparents affected are going to be Hollywood industry bankers and backers in the 1950s, big money types. I'm going to use William Holden's Mount Kenya Safari Club, or a slightly earlier version thereof, as the site where the viral species jump first happens over dinner. And Nic, bless him, sent me all the info about how the vampires can find instances in their own cultural lore about earlier outbreaks.