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.
How does it work? It's as simple as building passive immunity against diseases by allowing yourself to become sick in the first place. Chances are, you're eventually going to fall into the clutches of said disease at one point of your week/month/year/life. Why not take on the disease when your time schedule permits it, therefore allowing you to go to those business meetings meetings on vacation days. Rock on, buddy!
Because we know how much you love work.
Eat Your Nasty, Nasty Nasal Mucus
Hookey smackled pickles! Yeah, I'm being serious. Here's something you've probably never thought of before; why throw away your nasal excrement when you can eat it? While it may sound like a crazy, disgusting, idea, it can actually help you build a passive immunity comparable to a extremely small vaccine given by your doctor. Instead of going to the doctor's office every few months or so to get a horrific shot, you can simply transfer your nose mucus into your gaping mouth hole.
"Care to share some of those delicious nose fluids?"
For more reasons to eat your mucus, please consult this article.
Get Enough Sleep
Woah, this must be the craziest idea on this entire list! Actually, without doing this simple task, any passive immunity you may build up, will be rendered useless. While you may be a busy man/woman with big ties and fancy suitcases, you won't be able to conduct much business if you're incapacitated by horrific diseases. The best thing is, it's as simple as getting a full night's rest.
This dog got the right idea.
How does it work? Sadly enough, one effect of sleep deprivation is a weaken immune system. Any passive immunity that you may have built up throughout your entire life would have been thrown away. You'll have the tools and weapons to combat the intruders, but your troops would be half-awake to do anything about them. Sleep helps regulate, re-coordinate, re-sensitize, and re-balances hormones responsible for keeping your immune system in healthy shape. (Along with hundreds of other things) If you have insufficient sleep, you might as well kiss your immune system good bye and roll in a bunch parasite-infested mud.
"Welcome back, friend."