There are petting zoo animals at FW and at AK park. Plenty of people stay at the resorts and visit the kennel 2-4x daily to take their pets out for a walk. What about horseback riding at FW? I wonder if someone just got fleas on their clothing this way and then they brought them back to their room, the fleas reproduced and there was an infestation. It's not necessarily due to someone's misbehavior or lack of cleanliness. This also brings to mind the issue of baggage handling where luggage is piled together in hotels, shuttle buses, airports and airplanes.
It's really hard to be sure where someone gets a flea infestation. Common fleas don't live on humans at all. They live on animals. All it takes is a visit from an animal or very close contact to get them in your home. They normally "feed" on an animal (dog, cat, mouse

, squirrel, bunny, etc.) then get off the animal to lay eggs on a stationary surface. When the eggs hatch, the young fleas eat tiny bits of animal or human skin that accumulates on the ground/carpet near where animals/people live. Once mature, they need blood nutrients to reproduce, so they look for an animal (dog, cat, squirrel, mouse, etc.) to live on and bite. There is a lag in time from the egglaying until the offspring are looking for a blood meal. A common time for people to discover a household infestation is right after they return home from a trip. The eggs laid in the home prior to the trip are now hungry young adults. With no pet around to feed off, they will "settle" for the humans who return to the area and attack before getting away to lay their eggs!
All of this is to say, that the people themselves don't get infested. Their belongings may get infested or their home or hotel room. Disney will not likely own up to their responsibility for this problem unless they get complaints
while someone is actually in the room. If they learned of this problem in a more timely manner, they could have relocated the family and de-fleaed the room. If the family didn't have a problem until they got home, it probably was not fleas at the hotel.