The idea of the blood-drinking aristocrat in Transylvania (now Northwest Romania), appears to coincide with the arrival of the Ottoman Turk in that region.
For some reason nobody seems to make the obvious connection between the blood of Christ received at the holy eucharist and the feudal overlords who were the guardians of Holy Orthodoxy in that place and time.
As for the Toxoplasma Gondii parasite, we have lived with that ‘pandemic’ for several centuries. Our immunity must have built to accommodate the worst of it and only occasionally does TG make someone seriously mad. But what of the first people to have encountered domestic cats? How long might it have taken for us to gradually acquire some resilience to the parasite they carried?