The Hybrid Disease "Gift" punishes miscegenation between two different races that fulfill one of two different conditions, being in the same category, and having different Lifespan gifts. Next to all racial pairings that are safe from the second condition still bump into the first one, as nearly all categories have their own special lifespan gifts. The only two exceptions are Human on Anything Else, and Elf on Drakonid, and I'm pretty sure that last one is an oversight in their weird agenda.