You test as much as possible and for any positive you ask the candidate and all associates to self-isolate. Maybe X who is infected gives you a false negative,

You test as much as possible and for any positive you ask the candidate and all associates to self-isolate. Maybe X who is infected gives you a false negative, but if you also test X's mother, husband and son you are likely to get a positive and just isolate the whole household. Similarly a false positive just means you isolate one for no reason - no harm to the community.