I've had excellent results with Emergen-C and a very specific strength of Olive Leaf Extract herbal supplement.
Emergen-C is fairly straightforward.
Most Olive Leaf Extract is nowhere near strong enough to do you any good, but if you can find 500+mg tabs standardized to 15% Oleuropein, the results can be surprising.
I take 2 tabs at the first symptoms of illness, continue with 2 every 3-4 hours until I feel better. If already sick, take 3 tabs every 3-4 hours the first day, drop to 2 after that if need remains.
This stuff will upset your stomach a bit, but much less for me than many pharmaceuticals do.
Please note that I am not a medical professional, and that I have no formal medical/herbal expertise. I'm just sharing what works for me. YMMV
I was first given Olive Leaf Extract in the correct concentration by an herbalist/accupuncturist. Went in for an unrelated treatment with a raging head cold and fever. She took one look and broke out the Olive Leaf Extract.
My symptoms decreased dramatically with 30min of taking the pills and vanished altogether by the time I went to bed. It doesn't always work that rapidly or starkly, but it's kept me healthy for more than a year, now. May be worth a try.