An Air Travel Lemons Problem

I wonder whether allowing airports and gate agents to deny entry and travel to those ticket-holders who are coughing and sneezing all over the place could prevent a pretty nasty lemons problem.

Specify that risk aversion about contagious disease correlates with riskiness: the risk averse are more likely to take actions that prevent their being infectious. They'll wash their hands more regularly. They'll be careful about coughs and the like. And they'll be a bit quicker than others to avoid places that look risky.

Prior to any pandemic, only Howard Hughes refuses to fly. It takes extreme risk aversion to not get on the plane. So the airplane's population mirrors the general population.

When the pandemic hits, the most risk averse and consequently least risky take a look out the window and decide not to fly. Being stuck on a plane for an hour next to someone who is coughing - the risks of that are just too high for them, even though the absolute risk of being in that situation aren't really all that high.

When they leave, the remaining passengers on the plane are a bit riskier than the general population.

That makes flying a bit riskier than it otherwise was.

That makes the next tranche of risk-averse people avoid travelling.

And the whole thing unravels quickly from there until the plane only contains people who are coughing, or the risk-preferring. And air travel collapses.

The unravelling is less likely to happen if people expect that the gate agent would deny entry to someone who is coughing, and that the airport would actively be kicking out sick people.

It only works if people are more worried about being seated next to a cougher than about being mistakenly kicked off a flight. But I'd be pretty happy to be denied boarding for several flights to avoid the risk of being seated next to a sneezing person for one flight; perhaps I'm just among the more risk averse.

Is there any law or regulation that would prevent airlines from actively denying entry? Can they change their conditions of carriage to allow this discretion? Is it possible to block the unravelling?

Update: Note that this also applies to public transport. But at least you can get off of a bus if someone is sneezing.