You are right in that people go to work, to school on a plane, to the mall, etc. when not feeling well. Each one of those cases can result in exposing others to infection, but it is a fact of life. I do not think that most people who have saved and planned for a trip would cancel if they found they weren't feeling well the morning of the trip. Tom's advice is good. Invariably there will be someone on any given cruise that came aboard ill, or fell ill shortly after boarding. Being proactive is the only line of defense. (Of course, I contend that my alcohol consumption on board aids in my defense - LOL).