BANGKOK—Scientists have discovered a new type of the virus that causes a centuries-old pestilence, dengue. The surprising find, announced at a major dengue conference here today, is bound to complicate efforts to develop a vaccine against a tropical disease that is becoming a more pervasive global menace. But it could shed light on where the pathogen came from and whether it is evolving into a greater threat. The finding “may change the way we think about dengue virus evolution and emergence," says Duane Gubler, a dengue expert at the Duke-NUS Graduate Medical School in Singapore.