They attempt to keep if off the ship with medical forms. However, it was evident on that cruise that someone lied. The Mickey pool was closed for cleaning before the ship ever left Miami. A couple days later, I encountered a conversation in the hallway that made it clear someone tried to mask their symptoms and lied on the form and brought it on board. The bug hit primarily kids, presumably those who had been in the pool or club with the germ carrier.
Once it was evident that there were multiple people on board with it, they put all self service food on lockdown. You could not serve yourself at buffets (including Palo brunch), soda stations, or the ice cream station. The ship's crew worked max hours sanitizing EVERYTHING including wiping down the pages in the DVC sales book that was on display. CMs in stores began asking for room numbers instead of swiping room keys, to eliminate a point of possible germ transmission (keys remained in our hands), and just used our security picture for verification. There were wipes posted in EVERY common area (Shutters, store check outs, guest services), rather than just at the dining rooms. When we purchased drinks, we were handed just the receipt and a pen, rather than the receipt cover that normally gets passed among thousands. Every common item (ketchup, salt and pepper shakers, etc) was removed from MDR tables. Servers fist bumped instead of shaking hands. Hand sanitation upon leaving and returning to the ship in ports was enforced (more of a suggestion on day 1 than enforced). We knew which rooms were sick rooms because everything (room service trays, bedding, etc) was placed in bright yellow bags to alert employees to the biohazard. Stateroom hosts were gloved, aproned and masked when cleaning sick rooms. Those who were sick were quarantined. Public bathrooms were being cleaned frequently. If someone threw up in a common area, they were directed immediately to their stateroom and CMs got on the phone to report it immediately to the stateroom host. The area where the vomit had been was sanitized, complete with a black light inspection. On the day the ship returned to port, the rooms that had been sick were marked with checklists where employees had to note the time that the linens were changed, the room was sanitized, and when the room was fogged. We even got to see the hazmat spacemen that last day. They also delayed boarding the next cruise by a bit so that they could thoroughly sanitize.