The following is excerpted from Dr. Mercola’s article about swine flu which can be read here:
http://articles.mercola.com/sites/articles/archive/2009/04/29/Swine-Flu.aspx

Dr. Mercola has some controversial opinions about health and I don’t agree with everything he says, but he does recommend some helpful nutritional strategies for keeping one’s immune system strong so you are less likely to acquire infection. Here is his advice:

Optimize your vitamin D levels. As I’ve previously reported, optimizing your vitamin D levels is one of the absolute best strategies for avoiding infections of ALL kinds, and vitamin D deficiency is likely the TRUE culprit behind the seasonality of the flu — not the flu virus itself.

Avoid Sugar and Processed Foods. Sugar decreases the function of your immune system almost immediately, and as you likely know, a strong immune system is key to fighting off viruses and other illness. Be aware that sugar is present in foods you may not suspect, like ketchup and fruit juice.

Get Enough Rest. Just like it becomes harder for you to get your daily tasks done if you’re tired, if your body is overly fatigued it will be harder for it to fight the flu. Be sure to check out my article Guide to a Good Night’s Sleep for some great tips to help you get quality rest.

Have Effective Tools to Address Stress. We all face some stress every day, but if stress becomes overwhelming then your body will be less able to fight off the flu and other illness.

Exercise. When you exercise, you increase your circulation and your blood flow throughout your body. The components of your immune system are also better circulated, which means your immune system has a better chance of finding an illness before it spreads. You can review my exercise guidelines for some great tips on how to get started.

Take a good source of animal based omega-3 fats like Krill Oil. Increase your intake of healthy and essential fats like the omega-3 found in krill oil, which is crucial for maintaining health. It is also vitally important to avoid damaged omega-6 oils that are trans fats and in processed foods as it will seriously damage your immune response.

Wash Your Hands. Washing your hands will decrease your likelihood of spreading a virus to your nose, mouth or other people. Be sure you don’t use antibacterial soap for this — antibacterial soaps are completely unnecessary, and they cause far more harm than good. Instead, identify a simple chemical-free soap that you can switch your family to.

Eat Garlic Regularly. Garlic works like a broad-spectrum antibiotic against bacteria, virus, and protozoa in the body. And unlike with antibiotics, no resistance can be built up so it is an absolutely safe product to use. However, if you are allergic or don’t enjoy garlic it would be best to avoid as it will likely cause more harm than good.