look after it
By making your gut bacteria and fungi (flora) as healthy as possible. This means a good diet.
Your gut flora health is DIRECTLY related to your health . The gut is the first line in your immune system. You have to make it as healthy as possible.
To do this, you need a diet high in fiber and low in sugar.
Fiber is stuff that is not digestible by you. There are two typos of fiber. Insoluble fiber is what you end up pooping out. It's imperative to promote proper usage of the colon. Soluble fiber are compounds that you cannot digest but your flora CAN.
If you eat sugar, you are fertilizing the wrong flora. Bad bacteria and bad fungi will result in IBS and a poor immune (not to mention nasty gas!). Sugar is made from glucose and fructose. It is tbe fructose that is the problem.
The problem with fructose is systemic and not what you asked, but very related.
To BOOST your immune system, take a daily probiotic. It's the ONLY proven method of boosting the immune sys.