I seem to be immune against getting the flu

For as long as I remember I’m immune against the flue ... any types of flue and I would like to know what in my body is giving me my instant immunization ... as an exemple I would walk pass a person who has the flue ... or is not yet aware they are incubating the disease, and I would catch their flue bug and my incubation period would be just a couple of seconds or less and I would be “instantly” immunized ... thank you!