Conquest and Pestilence is interchangable depending on what you believe in. The White Horsemen for most religious folks represent Christ, thus he wears a Crown on his head to go out and conquer. Also the Color of White representing purity. But some see it as Disease/Pestilence/plague since it does conquer the human race symbolically by attacking the immune system making people sick all the world and usually spreading out in a continent infecting more people, thus "Conquering." The Hospital doctors and nurses who tend the sick seem to wear white as well. So you see the similarities.