Unless they find you beyond this world.
Get Sick More Often
Don't worry, that's no typo. If you want to boost your immune system, don't fall into the social-norm of avoiding as much microbes as possible. Instead, expose yourself to as many diseases (non-fatal ones, obviously) as possible, in order to build up your passive defense. Do you enjoy sanitizing your phones and door knobs every weekend? Of course not! Just don't do it and embrace the disease! It's like taking the disease head-on, to settle the score once and for all.
Depicted: Being weak-sauce.