Now you are changing the question. The rules are different if the exposure source is a CM than it is for another guest. Disney is
vicariously responsible for its employee's negligent conduct. It is not vicariously responsible for guest conduct. (Note I'm assuming that a CM would be negligent merely by showing up for work with COVID-19 even though they are asymptomatic. But I doubt a court would say the CM was negligent.)
I suspect the chances of isolating the source of the spread dramatically decreases with population size and relative guest anonymity.
You believe contact tracing is a great medical tool. I agree, for what it is worth (I don't claim medical knowledge). But not every great medical tool lends itself to proving legal causation. For example, the CDC describes contact tracing as primarily meant to look for people who were exposed to a source.
- In contact tracing, public health staff work with a patient to help them recall everyone with whom they have had close contact during the timeframe while they may have been infectious.
- Public health staff then warn these exposed individuals (contacts) of their potential exposure as rapidly and sensitively as possible.
https://www.cdc.gov/coronavirus/2019-ncov/php/principles-contact-tracing.html
I suppose it could be used the other way - to find out where an exposed person was exposed. Even so, that seems to me to require access to data that most lawyers will not have.
Take the wheelchair injury lawsuits I've written about previously. A guest claims another guest in a wheelchair or
scooter ran over them while at WDW. They sued WDW to get the identity of the other guest. It was a tedious process and the injured guest could, at least, say where the incident occurred. Disney eventually coughed up the guest's identity and was dropped from the lawsuit.
Here, I doubt WDW will be willing to release the names of all its guests over the course of another guest's vacation period. Disney refuses to release attendance figures and routinely asks courts to prevent these numbers from being publicly available. But even if a court ordered Disney to produce guest identities (which is highly doubtful), how would the lawyers use the data? There's no publicly available database of COVID-19 positive individuals (so far as I know). Also, the CDC says contact tracing should not identify the source by name. So, short of getting a very lucky hit on social media, do lawyers sue all the other WDW guests to find out if they were positive or later tested positive? If they did, they would probably be sanctioned for filing frivolous lawsuits.
Again, right now, I don't see a court holding Disney vicariously liable because it let in an asymptomatic guest who had COVID-19. I don't know of a legal principle that permits holding a company like Disney vicariously liable for a guest's conduct. Disney is responsible for its own negligent conduct. So short of Disney staying closed, I don't think courts are going to say Disney is liable even if one guest could prove another asymptomatic guest caused them to contract COVID-19.
Disney could be liable if someone could prove it let in a symptomatic guest. Again, I'm not sure how a lawyer would obtain proof of this but they can try.
The really interesting question is what will it take before courts will hold companies like Disney, Universal or even the Chicago Symphony Orchestra liable because they decided not to conduct COVID-19 testing on guests or visitors. That question fascinates me because it is a standard of care (or duty) question. The standard of care in tort law turns on utility or relative cost. As one court put it, the standard of care balances the relative harm against the utility of the desired result (this can be a financial cost or personal preference).
McMahon v. Bunn-O-Matic Corp., 150 F.3d 651, 659 (7th Cir. 1998) ("without evidence that a holding temperature of 180° F is of little worth to consumers, plaintiffs cannot show that the choice of a high temperature makes coffee defective").
So if you want good tasting hot coffee - meaning coffee held at 180 degrees (slightly hotter than the industry standard 170 degrees), you take the chance that you might get hurt worse than if you limited your custom to shops that served coffee at 140 degrees. Contrary to popular belief, courts have actually held that serving hot coffee does not make the coffee (or the coffee making machine) inherently dangerous.
Coffee is not the greatest analogy. The severe health risks associated with contracting COVID-19 will likely warrant a greater duty. But there is still a need for balancing the risk against utility. When testing becomes more available, I would expect courts will hold Disney and other companies to a greater degree of care, i.e., the company's liability will increase if it does not conduct readily available accurate tests.
Now, let's take this positivity to another even more absurd level. We'll assume that as of July 1, 2020 (just in time for the July 4 holiday) accurate tests for COVID-19 become readily and cheaply available. Disney then mandates all guests take the test (increasing ticket prices to pay for the test, of course) before entering a park. Where will Disney do the tests? There aren't exactly private locations avaialble to all guests. Even if WDW conducted tests at the resorts, lots more folks stay off property. I suspect WDW would have to test guests before they leave their room to board a Disney bus or, for those that don't take the bus, when their vehicle arrives at the parking lot or drop off location. That sounds like a logistical nightmare but its theoretically doable.
So what is Disney's obligation when a guest tests positive? Sure, WDW excludes that guests, and almost certainly all the other guests in the same "party." But what does Disney do (what must it legally do) with other guests who are not in the "party" who may have been standing near the positive guest or who may have come in contact with that guest?