Of all the disease-spreading insects in the world, the mosquito poses the greatest menace, according to the World Health Organization. As if to underscore that threat, two mosquito-borne viral diseases have begun to spread well beyond their points of origin. One is dengue fever, a potentially deadly illness, and the other is chikungunya, a debilitating and painful disease from which most people can recover. There are no vaccines to prevent these diseases. But researchers are working hard to develop vaccines against dengue fever and chikungunya, and to control the mosquitoes that spread them. VOA's Carol Pearson has more.