I doubt can you upgrade your immune system in just a few days. But, it can't hurt to get more sleep, plenty of water, daily exercise (even walking 1 mi), and cut out all junk foods for a few days (no sugar, no hydrogenated fats, no fried or fast foods, no processed flour/especially wheat, minimal red meat, etc. Then get as many anti-oxidants as you can (vitamins, minerals, etc.). Easy ones would be through berries/blueberries/strawberries and citrus fruits. Quinoa, tomato sauce, beans, garlic and onions, spices/tumeric, veggies/leafy greens and cruciferous veggies are also good sources of anti-oxidants. In the days of the plague wearing garlic was more than a fashion statement. Vitamins A,B,C,D,E, selenium, CoQ10, are other good things to get...probably through a quality multi-vitamin. Usually what you find at the supermarkets or big box stores are not good vitamins. 1000-2000 mg Vit C each day might help. Your green tea is a good source of anti-ox. I drink 2 cups cold during the day. Even 1 oz of 80-100% cacao dark choc per day is a good source of anti-ox.
Follow a good regimen for a couple of months and say good-bye to catching colds, flus, bronchitis, and other common winter sicknesses. I haven't had any of those in over 6 years since I changed my diet...and I was getting hit several times a year before that. I don't take flu shots either. Your quality immune system can handle most anything, at least until you get up there in years.
For those who shun the sun or live in cold climates, your immune systems are depressed by Nov-March of each year as your Vitamin D stores have been depleted (half life of approx 4-6 weeks). That's the majority of the USA...NJ included. If I couldn't get UV exposure from sun light or equivalent, I'd be taking a 4,000-8,000 IU Vit D3 supplement every day. I don't think anyone can have a solid immune system when their Vit D levels are depleted. The draw back of "avoiding" skin cancers via minimal sun exposure, is that your Vit D levels are minimal as well. That invites a whole host of other ills. NJ in March? Unless you are doing something specific for Vit D, you're depleted and your immune system is ratcheted down a few notches. Still, it takes weeks/months to get your Vit D levels boosted on your own
Eating junk and big meals adds to the stress on your body or immune system. I'd eat well, but in small quantities. I also wouldn't do any major league exercise routines (like 5-10 mile runs or 20 miles bike rides) until this flu passes. If you stress out your body from food, exercise, lack of sleep, 10-12 hour days at work, etc. it just means you've increased the odds of getting the flu.
