Four global epidemics over the last century have caused many deaths from complications among people with weak immunity, such as old people and young children.
流行性感冒是一種急性呼吸道傳染病，過去一百年來曾發生過四次世界性大流行，造成許多免疫力較差的老人及幼童因併發症而導致死亡。