Hello clauee,
Most of your questions are answered by the fact that serological tests don’t perfectly detect antibodies that are only caused by the presence of the disease you are testing for. These tests cross-react with other antibodies or in some disease’s antibodies may form during exposure, but this individual resisted the disease. Perhaps there are related organisms that don’t persist but cross-react. Then there are the individuals who were infected and then successfully treated (B caballi) but the antibodies persist. There is no test that anybody would claim is 100% specific for the presence of infection but currently, the most accurate test would be PCR where they look for pieces of the organism’s DNA. Cornell runs this test I believe.
DrO