There are certain illnesses in which the diagnosis is quite difficult. When the disease has few symptoms, figuring out what is going on can be puzzling. When the disease causes symptoms which are found in many other illnesses, diagnosing the right one can be troublesome. And, when the disease has already been successfully destroyed by the body's own immune system, deciding on what caused the illness in the first place is often arduous. So, a common technique physicians use in these situations is to look for the antibodies the body produces to fight off the illness.
Antibodies are proteins which attach to the invader so that the body can identify and kill it.
When we can't test for the actual invader, we can look for the body's response to that invader. But when a specific antibody is found to be elevated, it does not necessarily give doctors a clue as to the correct diagnosis. It all has to with timing. A nice example of this is with chicken pox. After a person has chicken pox, the antibodies against chicken pox remain elevated for life.