One is quasi-active, like the flu example I gave, where we get some people to nominate their friends and then passively monitor their friends -- do they have the flu, or not? -- and then get warning.
另一种是类似于主动的方式， 比如说像我说的流感的例子， 我们选一些人来举出他们的朋友 然后被动地监测他们的朋友 --- 他们是不是感染了流感？ -- 然后得到示警。