Falling sick can be quite unpleasant for a wide variety of reasons, and you would do well to look into avoiding getting sick in the first place. If you want to stop falling sick you need to prop up your immune system. Our immune system is quite strong and a big part of the reason why that is the case has to do with the fact that it has been designed by the body to be as strong as possible since this would ensure protection from infections. However, what you also need to realize is that sometimes your immune system might not be working at full capacity or it might be faced with a certain kind of foe that is far beyond anything it has ever faced before thereby making it very important that you end up giving it as much help as it needs at any given point in time.
Checking out 4life research can shed some light on several aspects of this but the gist of what we are trying to say is that you need to boost your immune system and the best way to do that is to consume citrus. Orange juice is a great source of nutrients all of which would contribute greatly towards giving your immune system the kind of boost that it actually needed.
You can also try adding lemon to a few things here and there. Note that cooking fruits destroys a lot of the nutrients that you would have wanted so try as much as possible to eat raw fruits since they are the most nutritious. Eating citrus fruits as often as possible would help make things easier for you in the long run.