-Airlines have cut hundreds of thousands of flights because of coronavirus travel restrictions.

-Carriers also are waiving change and cancellation fees for some routes because of the illness. 

-U.S. domestic demand has been a strong revenue-driver and will now be put to the test.

The rapid spread of the new coronavirus is testing airlines and other travel companies with a risk that had been nearly unthinkable over the past decade: a broad decline in travel demand.

Air-travel demand had been growing at twice the pace of the global economy, but that bright spot is now at risk. U.S. airlines and other travel stocks have tumbled more than the broader market in this week's rout as big conferences were canceled and fears grew that customers may just opt out of trips because of the spreading COVID-19 outbreak.

The issue caps a difficult year for airlines that have been grappling with the nearly yearlong grounding of the Boeing 737 Max. Carriers need demand to stay robust, particularly in the lucrative spring and summer travel seasons, and analysts are warning that that looks unlikely.