If that was a possibility, I think we'd already have seen lawsuits over DL and WDW-linked measles outbreaks or other communicable diseases. The general legal presumption in our country is "at your own risk" when it comes to public places and contagious disease. Unless there's a specific failing that causes the disease (ie, foodbourne illness that can be tied to violations of safe-handling practices or maintenance deficiencies leading to a Legionnaire's outbreak), public places aren't liable for illness transmitted between customers. I don't see that changing because of this or we'd have no more theme parks, concerts, malls, gyms, etc.