Viruses often cause massive failure in staple crops in developing countries. Two years age, Africa lost more than half its cassava crop -- a key source of calories -- to the mosaic virus.

• 病毒常常在发展中国家造成主要粮食作物的大面积歉收。两年前,花叶病毒使非洲损失了超过一半的木薯,而这种作物是当地人的主要食物。