Saiyuki

From Tokyopop:

Having been assigned by the human elders the mission of stopping a mystical plague that has afflicted the Youkai with madness, Genjyo Sanzo assembles his team of Youkai warriors, desperately hoping that the disease will not affect them. He sets up qualifying tests that will help him determine the loyalty and worth of Cho Hakkai, Son Goku and Sha Gojyo. The team then journeys west to rid the land of madness.