I don't know these terms, but If I am understand this correctly:
>only 1 in 1000 people have the disease
>rate of occurrence 0.1% in the natural population
>a false positive rate of 5% means, that if you give it to a RANDOM person, they have a 5% chance to score positive on the test
>So if you gave the test to 1000 people, 51 people would score positive (the 5% false positives and the 0.1% of the population, who actually has it)
>So that's a 1 in 51 chance you are the person with the disease in purely mathematical terms
>that's like 1.9%