What if biological weapons existed which selectively killed people of particular nations, races or religions?

It's an interesting idea, isn't it? I think the closest thing to this situation was the extermination of the native peoples of the Americas by European diseases during the period of European colonization of the Americas. For the most part, this was by accident, of course. Because the native people's of the America's came across the Bering Land Bridge, the conditions were so harsh that no disease organisms could survive the passage, so most Euroasian diseaes like smallpox and inlfuenza simply did not exist there. As a result the native people had no immunity, and were exterminated in huge numbers -- estimates ranging from 80% to 98% of the native population died. Now, this may not have been entirely the effects of the diseases, of course. Simply having everyone in a village sick for weeks may have been enough to cause everyone to die of thirst and hunger, with no one healthy enough to collect food, or water. In any case, the extermination of the native peoples of the Americas was a tremendous boon to Europeans, suddenly catapuling them to world dominance in the eighteenth and nineteenth centuries, particularly.