Hand washing does more than any other single technique to prevent person-to-person transmission of germs. Keep your nails short and clean. Dry with an absorbent cloth to remove germs not removed by washing.

Use a biodegradable soap or hand sanitizer when washing your hands while camping.
Do not share personal utensils, water bottles, etc. Do not reach into food bags with your hands
Keep anyone with the slightest indications of a cold, flu or skin infection out of the camp kitchen.
Wash and disinfect camp kitchen gear daily. An iodine based product, such as Potable Aqua, adequately disinfects water for rinsing kitchen gear, using less time and less fuel than boiling.
Take breaks and camp away from trails and other visitors.
When "nature calls," select a place that is at least 200 feet from a water source. Choose a place with
    1. Heavy soil as opposed to light soil,
    2. Dry ground as opposed to wet ground and
    3. Flat terrain as opposed to a slope. Dig a hole 6 to 12 inches deep and then bury it.