One way to look at this is to imagine a jar filled with water. If you add a single grain of salt, you can still drink the water, it is healthy. But if you keep adding salt at some point the water is salt water and it is not healthy for you. Your job [if you choose to accept it] is to keep refreshing the best fresh water you can into your "jar" so that the grains of salt are never numerous enough to make salt water.
Your immune system is like that jar, it is either filled with good guys or bad guys. Which ever is in the majority determines your state of health. Basically the more good guys you have in your immune system the less room there is for bad guys. The good guys keep the bad guys in check.
It is not necessary to kill all the bad guys with drugs that over power the body. It is only necessary to have more good guys than bad guys to keep them in check.
By Reverend Alan (talking about eating good bacteria ie probiotics, yogurts and stuff)