The most effective thing that I've found is pressed, unfiltered apple cider. At the first sign of the onset of a cold or flu, drink at least half a gallon in an 8 - 12 hour period, and within 12 hours following, the symptoms are drastically reduced or gone. I have tested this remedy on myself, family and co-workers and it works every time.
How does it work?
1) "An apple a day keeps the doctor away," would be the simplest way to say it. Half a gallon of cider has the nutritional impact of 12 apples in one day.
2) Nutraceuticals 101: Fruits and vegetables resemble the human organ that they target. Apple cider resembles blood plasma. It targets the white blood cells - reinforces your body's immune system. (Note: filtered clear apple juice resembles urine. It will make you pee, and that's all it's really good for.)
3) Acid/Alkaline balance. Our diets are largely acidic. Bacteria and viruses thrive in an acidic environment. Apples are highly alkaline, and when you drink so much cider at once, you shift your biochemistry to alkaline, which bacteria and viruses, generally speaking, can't survive.
All that, and it tastes a heck of a lot better than raw garlic, which, if you're not careful, will burn your esophagus.
Apple cider is typically found in the produce aisle, or refrigerated juices section at your local supermarket. I haven't missed a day of work due to illness since I discovered this remedy, and neither have my co-workers.