Vitamin C is a great immune system booster. It's also a vitamin that you really can not ingest too much of - ok you could, but it would mean crazy amounts. Vitamin C is water soluble and is therefore pretty safe to increase your intake of without much concern. I once read Dr. Andrew Weil say that he recommended folks who felt a cold or flu coming on supplement with Vitamin C until they felt flatulent. This would be the amount of C your body can process and hopefully the maximum immune benefit. Then, stop supplementing or you will go from there to diarrhea. Also, because it is water soluble, spread your intake throughout the day. Otherwise, you will eliminate the excess with fewer benefits.
But why supplement?! You can eat your Vitamin C all day and it'll be tastier! Vitamin C fruits and veggies are amongst my favorite. Right now, is not a good season for our Vitamin C filled fruits - they like it warm. Most of us know about oranges and grapefruits, but kiwi are even higher in Vitamin C content. Strawberries are also high, so maybe some frozen berries in a smoothie are in order? Vitamin C filled fruits are very acidic, so spread them out or you'll wind up with a sore mouth. If you plan to drink your Vitamin C, make sure you've got limited sugar content (so, it's not the first or second ingredient- sucrose, dextrose, corn syrup, etc - on the label).
Right now is a good time for Vitamin C filled veggies, however! Broccoli, and Brussels sprouts are good Vitamin C sources and perfect side dishes this fall. Butternut squash is a soup favorite and is delicious in curries. Bok Choy can be yummy in a stir fry (broccoli as well!). Bell peppers are one of the highest Vitamin C veggies and can be added to just about everything you prepare as a meal or a snack. Kale and Swiss chard can be chopped and eaten as greens, in rice dishes, soups, and pastas.
Finally, there's our fruit/veggie - the tomato. It IS a fruit, technically, but the government called it a vegetable way back, so it could bypass taxes on fruits and now it's stuck with the misnomer. It really doesn't matter if this one's in season, because cooked tomatoes with some olive oil mixed in, have been found to be more nutrient rich to the human system than raw (especially, those mealy, colorless ones available in winter months). So, make your favorite soup, sauce, etc. all winter and keep the Vitamin C plentiful!
Don't just focus on the Vitamin C when you feel sick, keep this focus throughout the cold and flu season. By easily incorporating Vitamin C rich foods or juices to your 3 meals a day, your immune system will get the boost it needs to keep you healthy.