[jerome-jh]

Maybe not the answer you are expecting, but the true way is to expose yourself to diseases.

D vitamin may help: you best get it by exposing your naked skin to sunlight.

[fefb]

Yeah . Nice add. I believe I already was exposed to differents diseases. I grew up in a third world country. Also, I did a exchange program in a multicultural US University where I had contact with different people from around the world.

I will try to expose more to sunlight , arms and legs. I spend all my day indoor.

[silicon2401]

I will add on to this that you can get enough vitamin D via diet, whereas exposure to sunlight is one of the primary contributors to aging.