I have been taking it for about 18 months now as maintenance. In that time I haven't been sick. When my wife first started taking it she was in her first year of primary school teaching. She would catch something at least once a fortnight and at the 3rd term mark (4 terms in a school year) she was sick for with the flu for 10 weeks straight.
Mms fixed her up and she hasn't been sick since and that was about 10 months ago.