We took 1,300 randomly selected undergraduates, we had them nominate their friends, and we followed both the random students and their friends daily in time to see whether or not they had the flu epidemic.
我们随机选择了1300本科学生， 让他们推举他们的朋友， 然后我们跟踪随机的学生人群和他们的朋友 每天按时 观察他们是否传染上流感。