Lets say you have a small group of space colonists. They were screened for sickness, viral and bacterial, as thoroughly as we knew how before being sent off to the colony and all were found healthy. They traveled for years in a fully sanitized ship before reaching their new colony, which is isolated from all other humanity, or other earth-animals. Effectively there is no one to catch a cold from on this new planet.
How long would it take for sickness and disease (viral and bacterial, not counting things like cancers) in this population? What length of time would it take for the equivalent of your common cold to pop up? How long would it take for more dangerous potentially life-threatening illnesses to crop up?