What happened

Shares of Carnival (NYSE:CCL) (NYSE:CUK) tanked in early trading Friday, and it wasn't long before the rest of the cruise stocks followed. By 3:33 p.m. EDT, shares of Carnival were down 5.2%, bracketed by Royal Caribbean (NYSE:RCL) with a 4.4% loss and Norwegian Cruise Line Holdings (NYSE:NCLH) down 5.6%.

It's not hard to figure out why. The answer is coronavirus -- and the threat of renewed lockdowns and other preventive measures it brings with it.

Collage showing a cruise ship, a person in a face mask and coronavirus particles.

Image source: Getty Images.

So what

An internal document from the U.S. Centers for Disease Control and Prevention (CDC) lays out some rather alarming language about the threat posed by the delta variant of COVID-19, describing it as as transmissible as chickenpox and warning that "the war [against coronavirus] has changed." 

And I'm afraid we're not talking about the good kind of change. Commenting on the memo, which was first reported on by The Washington Post last night, CDC Director Dr. Rochelle Walensky told CNN today, "I think people need to understand that we're not crying wolf here. This is serious." She further characterized the delta variant as "one of the most transmissible viruses we know about," adding, "The measures we need to get this under control -- they're extreme."

Now what

Suffice it to say that these kinds of warnings from America's premier source of pandemic healthcare advice do not bode well for the cruise industry -- or for any stocks connected with vacation and leisure, for that matter. Already, the CDC has issued guidance recommending that even vaccinated Americans wear masks indoors in certain situations, and called for universal masking in schools. If and when new advice is issued specific to the cruise industry, it might very well disrupt Carnival's plans to get half its fleet operating again by mid-October, as it recently promised -- and delay Norwegian's plans to have its entire fleet sailing again by mid-April 2022.

Now that delta is surging, that might not happen after all.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.