I absolutely agree with Asher Gray. Vitamin C is the way to go. I take 2 gms in the morning and 2 at night along with a Coral Calcium and a few tablets of Vitamin D3 every day. The combination works synergistically and helps your body get the most out of the C. I'm only 120 lbs so I don't need a higher dose and I am a very healthy eater and I rarely have anything makes me sick.
IF something has dampered my immune system (like too much sugar, some kind of toxin, or a messload of stress) and I start to feel anything coming on I up the C to 3 or 4 gms of the time release Vitmin C along with the Coral Calcium and D3 every 4 hours even through the night if I wake up. If I get gassy I drop it back a gram or so, if it needs more I up it. I listen to my body to let me know which way to go. But I keep up the dosing every 4 hours for at least 24 hours then drop it back to every 6 and then back to my twice a day over a few days. Don't stop the higher doses cold turkey, taper it off.
The most important aspect is that it truly is a situation of "A stitch in time saves 9". If I can catch the telltale signs early and take a high dose of C, calcium and D3 right away, I can stop it in its tracks. Again taper off, But if I happen to be out or away from C and it takes a bit of a hold before I get there. I need way more for much longer to kick it out.
For sore throats that sometimes come on. I also use Oil of Oregano. If you have any kind of sore throat it can taste pretty nasty and feel aweful for a few moments but it sure kicks the sore throat out fast. Especially when you add the C regime.
Lastly a soak in a hot tub with a cup of 3% hydrogen peroxide added does wonders.
Unless there is an emotional issue that I need to address that usually kicks any illness away. And for Emotional issues that need addressing I use EFT. http://www.thetappingsolution.com
Sometimes if I don't have C handy tapping can stop it in its track too.
Before I understood about C. calcium and D3 if I caught a cold it could last for a week or two. Now even if I get a right runny nose it is gone in a day or two.