No, Disney security is not analyzing people as they walk through park entrances to see if the "appear" sick. What do you suggest? They pull people aside and put thermometers in their mouths or actually test for flu? If someone has the full-blown flu, the chances of them having the energy to go to a park are slim to none. If they are in the early contagion stages, they won't even know they are sick (nor will they look sick) and, yes, they very well might be there. You could also catch a cold there or a stomach virus or any other number of viruses. The very same viruses you could catch at the grocery store, the mall, your workplace, etc. People throw up at WDW on a daily basis and not from the flu. You could always get a flu shot and wash your hands frequently or hermetically seal yourself in your house, but, no Disney is not scanning crowds and detaining people or refusing entry to people who "appear" sick.