Apparently earlier this week Disneyland was crowded as fuck despite the rain and the worry about Coronavirus, even though Orange County is in a state of emergency over it. Disney World I know less about but it's a given that the risk will be even worse since it's bigger and sees more international travel to the resort. I'm not surprised they're closing the parks because it seems like a lot of opportunists are taking this as a time to travel cheaply and with less people around, even if the main concern isn't necessarily the travelers getting sick but spreading the virus further. Having the parks open is an unnecessary risk IMO since not only are people irresponsible and going to try to go to Disney despite being sick/the risk of getting sick, but it seems someone can be contagious even before they know they have it.
EDIT: They're not looking to close Disney World and my post might imply otherwise but they really should for the stated reasons.