然而,目前为止最普遍的流行病杀手 However, by far the greatest pandemic killer 是流行性感冒。 is influenza. 流感持续不断地在南北半球循环。 Flu is constantly _____ between the Southern and Northern Hemispheres. 在北美和欧洲, In North America and Europe, 季节性流感每个秋冬季都会出现。 _____ flus occur every autumn and winter. 大多数的儿童和成年人在之前[br]都接触过流感病毒, As the majority of children and adults will have been _____ to the virus in previous seasons, 这些疾病通常是很温和的。 these illnesses are usually mild.