they changed how they diagnose it. they no longer use the PCR test run multiple times, because it guarantees positive results.. the CDC finally admitted this. they also are now requiring a test to prove you had it, whereas before they doctor simply 'had to feel like it was'.
they did something similiar with whooping cough. at first you only needed to have it for any length of time.
then after they introduced the vaccine.. suddenly they changed the diagnosis to requiring you to be sick for 7 days. well it almost never lasts that long anyway..
so whooping cough disappeared via how they diagnosed it.
here is a study where they tried to prove the flu was contagious.. but couldn't.