Take for example polio. The rate of occurrence among infected people is from one in 1000 to one in 100. That is to say, when a doctor sees one case of polio, he should immediately recognize that from 100 to 1000 people have been infected, and must on no account waste time by waiting to see what happens.
以小兒麻痺為例，小兒麻痺病毒感染的發病率是百分之一到千分之一，換句話說，醫生在看到一個小兒麻痺案例時，就該懷疑到：是否已有一百到一千人受到感染，絕對不能等閒視之。