For both the short term, to counteract germ exposure, and for long term immunity benefits, Vitamin C is a basic immunity supplement. Vitamin C has long been considered an important part of fighting a cold, but its proactive ability to prevent you from getting sick is also well established. For boosting your immunity, you should attempt to get Vitamin C in its purest form. Currently some nutritionists, who are working with cancer patients concerned about rebuilding their immunity while in chemotherapy, encourage getting vitamin C in high quantity from very fresh sources such as certain edible flowers. This is why you will often see rose hips associated with Vitamin C due to the high C content. (Wikipedia defines rose hips as "the aggregate fruit of the rose, a berry-like structure called a rose hip." Essentially, high quantities of vitamin C concentrate in these berry-like structures, and it is viewed as a higher dosage of vitamin C that can be better tolerated for consumption.) Purer, concentrated forms of vitamin C, such as rose hips or a form called C Ester (Ascorbyl Palmitate) are generally tolerated in much higher quantities in the system without side effects, so you can take more of it and derive more benefit. If when you are first exposed to illness, you take vitamin C regularly for the first several hours and then again the next day until the risk of your exposure is over, you most likely can avoid the cold or bug to which you were exposed. Vitamin C also works as an antioxidant, allowing your body to purge itself of waste and bacteria. For any use of antioxidants with the intention of reducing the effects of a specific exposure to illness, drinking plenty of fluids and antioxidant use helps flush the system of the germs, hopefully before any take hold in your body. Vitamin C with rose hips is readily available at drugstores and C Ester is found at Vitamin Shoppe and other retailers.
Antioxidants generally help the body function better, including the immune system - directly and indirectly - by aiding the lungs, liver and kidneys to help blood, oxygen and nutrients to be able to reach the tissues of your muscles and organs, helping them function optimally. Alpha Lipoic Acid is considered the wonder antioxidant; it is even being considered in studies for the treatment of HIV/AIDS. It helps purge germs from your system; in studies, it has killed germs on contact in laboratory testing. You can also take Alpha Lipoic Acid in higher doses, meaning you can take a daily dose for your overall immune health, and an extra dose upon being exposed to any illness.
Acidophilus is best known as what makes yogurt good for you (the "good" bacteria in your system) but to really address immunity problems, you need to take it daily. Best known for counteracting yeast infections in the system, studies show a direct correlation to Acidophilus and other probotics improving immunity. While for some people eating yogurt is enough Acidophilus, if you prefer not to eat yogurt, Acidophilus is also available in pill form. Acidophilus is especially worth considering if many of the times you get sick have a digestive system or gastrointestinal component. While it will likely not affect matters one way or another for an immediate expose to a specific illness or sick person, it will build your body's defense mechanism against bad bacteria by strengthening the good bacteria in your system. Recent studies indicate that yogurt is especially helpful to those on cortisone medications, such as asthmatics, to help them build immunity. Nature's Made makes an Acidophilus pill without any yeast or gluten, which aids in its effectiveness.
Green tea has a dual immunity-building purpose of working as an antioxidant and to strengthen your system and rebuild healthier cells. It comes as a liquid or supplement pill form, and a concentrated liquid extract. White tea and red tea both have more antioxidant properties than green, but are lacking the cell rebuilding properties. Green tea supplements work as an antioxidant for concerns over a specific exposure, giving you both the benefit of the liquid to flush your system and the antioxidant to fight germs. It also works to strengthen red blood cells, helping those with chronic illnesses or immunity issues gain strength, even while undergoing chemotherapy. Green tea on a long-term basis replaces damaged cells with healthy ones. For example, the intestine replaces its cells once a year, so if you drank green tea in that time, you would build a healthier intestinal system over the course of the year, giving you a better ability to fight illness. Natrol makes a reasonably priced green tea supplement pill that is easy to swallow.