If you test 1000 people with a test which has a 1% false positive rate (lets ignore false negatives), but only one person in the 1000 is really infected you get

If you test 1000 people with a test which has a 1% false positive rate (lets ignore false negatives), but only one person in the 1000 is really infected you get about 11 positive tests results of which 10 are wrong.