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.
Tomamos 1.300 estudiantes seleccionados al azar, hicimos que elijan a sus amigos y seguimos a los estudiantes elegidos al azar y a sus amigos diariamente para ver si tenían o no la epidemia de gripe.