[Other than what's for Buffy to tell about herself, he doesn't have a problem if people listen in on this. Best way, after all, to explain his own condition...without telling anyone he has it. Just in case.]

Two parts to it; the first is viral, like a sickness. The second is...a bit more impossible to explain. It's a sickness of the soul. One that ends with the soul being devoured completely.

You get it by being bitten by a vampire, or getting a scratch that draws blood. At least that's how it works on my world. There's a few other ways, too, but pretty much, it all depends on the infection getting into the victims bloodstream somehow.