If you have any kind of test that seems relatively accurate (5% failure rate), 1 in every 20 tests will be identified as a positive candidate no matter the actual incidence rate. When applied over a huge population the number of positive candidates is going to also be huge. Since the incidence rate in this scenario is so low (1/1000) the odds of a true positive selected from the positive candidates is also very low.
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If you have any kind of test that seems relatively accurate (5% failure rate), 1 in every 20 tests will be identified as a positive candidate no matter the actual incidence rate. When applied over a huge population the number of positive candidates is going to also be huge. Since the incidence rate in this scenario is so low (1/1000) the odds of a true positive selected from the positive candidates is also very low.