I think getting it 4 days ahead of time will make sure it probably will be gone during the first days there, if I had to choose it would be before so the worst was over before I left.
It's the season for colds/flus, when we boarded the plane in Orlando 9/16 (our departure date) my nose started running and I got all stuffed up, we got home and it all broke loose, I've been sick since Saturday night with chills, fever, the nose running...the all included in a cold/flu.
I'm just happy it happened after our trip, though I felt kind of dizzy at times on our last days (pre-cold/flu?) that was all at WDW.
DD had the stomach bug before I first trip, coming out both ends, imagine our panic (cancel/not cancel?) the day before we left we took her to her pediatrician, she gave her medicine against nausea and she stopped throwing up, all that night before going to the airport we stayed awake waiting for her to run to the bathroom but nothing happened, she was over it!
We left for our trip, she was a little "off" during the flight but once we got to Orlando and WDW it was like she had never been sick at all! Though I thought the stress of not knowing what to do/would she get better gave me an ulcer

Sorry if long
