As old barriers break down, voyagers in the next century will enjoy more exotic locales, more exotic customs - and perhaps more exotic diseases. This year's Ebola virus outbreak in Zaire raises an issue as chilling now as it was in 1347, when traders sailing from the Black Sea port of Caffa to Messina, Sicily, brought back plague, which killed perhaps one-third of Europe's population. As it happens, plague also cropped up in 1994, in India.