[Mvandenbergh]

Let's say you have 1000 people, 1% or 10 have had the virus and are seropositive (have antibodies).

Of the 900 people who do not have ABs, 99.9% or 899.1 are correctly identified as not having them, 0.9 is identified incorrectly as having them when they actually do not.

Of the 10 who actually have antibodies, 100% are correctly identified.

So 10.9 are identified as having antibodies, in 0.9 person's case incorrectly which is about 10%.