Definition:
A false positive test occurs when a person tests positive for a disease that they do not have. They occur because no diagnostic test is perfect.
Test quality is always a matter of balancing the need to try to catch as many cases as possible (sensitivity) with the need to not diagnose people who aren't actually sick (specificity.) Unfortunately, it's difficult to design tests that are good at both, so scientists try and figure out which outcome is worse in any given situation - a false positive or false negative and weigh things accordingly.
