I used to get colds two or three times a year every year. Then I started eating an orange a day (which seems to reduce the incidence of my canker sores) and I more or less stopped having colds and getting sick for that matter. I did catch one, one time but I would have one three times a day and I could function fine with just a little loss of energy. A few times, I would feel a sore throat coming on, eat an orange immediately and it would go away.
So it seems to work for me!
Personally, I think the vitamin C is part of it but also the combination of its trace minerals.
I started eating an orange daily in August of 2011.