All that healty immune building stuff is snake oil i think. Of course excersizing is good no doubt about it. The only tried and true thing to build the immune system is bad stuff, dirt, germs, bateria, etc. Bad things is what you need for immune system to build a strong defense against them. Look at our parents and grandparents. They played in the dirt, mud, played with frogs, snakes. did all that bad stuff thats bad for you. Old men are strong as an Ox. people nowadays are delicate and fragile. people try to shield themselves so much that when they are actually exposed to "bad" stuff thier body cant hang with it.
Im not pointing any fingers at the OP, I'm just giving a response to all the solutions that people think is a cure all.
If you honestly think you get sick to often maybe you can see your doctor and voice your concernes about it. Some people do naturally get sick more often than others. My wife gets sick 3x more than me at least. Obviously we are exposed to the same things as we live/eat/sleep together and go the same places, except for me going to work we are exposed to the same exact things, so IDK..