Instead of asking what makes you sick, rather look at what keeps you healthy.
General health tips:
Fruit, vegetables and white meat - Eating Healthy
Regular snacking (small portions) is much more healthy than large meals.
Take daily supplements (Multi-vitamins, especially Vitamin B1/6/12, Omega3/6) and also weak probiotics (like the children take) and CellFood generally keep your immune system strong.
Regular and ocassional intense exercise is also excellent and recommended.
Good and regular sleeping patterns (specifically at night - between 6-7 1/2 hours show to be most efficient)
Sanitation tips:
Keep the office and workplace clean and clean regularily - I read somewhere keyboards contain on average 60% more germs than toilet seats - clean those buggers! According to dentists - toothbrushes should be replaced every 1-2 months.
Make sure the drinking water is good and there is good ventilation.
And wash those cups!
Har har :) Hope these tips provide some value to you. Don't consider me a fanatic - I work in the Health Care industry (as a hacker/developer) and pick a couple of things up along the way.
And lbcadden3 is right - daycare is a germ factory.
Also, try not to have your co-workers sit/work closer than 1 1/2 or 2 metres in proximity of eachother - unless you are looking for the domino effect!