"Sudden illness" 用英语可以说作 "acute illness" 或 "sudden onset illness"。这个词组来源于医学术语，指的是突然发病且病情严重的疾病。
例句:
He was taken to the hospital with acute appendicitis. (他因急性阑尾炎紧急送往医院)
She was diagnosed with a sudden onset illness, and was rushed to the emergency room. (她被诊断出患有突发疾病，被急送到急诊室)