“And then I see the disinfectant where it knocks it out in a minute — one minute — and is there a way we can do something like that by injection inside, or almost a cleaning?” he asked. “Because you see it gets in the lungs and it does a tremendous number on the lungs, so it would be interesting to check that.”

Experts have long warned that ultraviolet lamps can harm humans if used improperly — when the exposure is outside the body, much less inside. The link between ultraviolet light and skin cancer is well established. Bleach and other disinfectants may kill microbes but they also can kill humans if swallowed or if fumes are too powerful. That is why bottles of bleach and other disinfectants carry sharp warnings of ingestion dangers.

Mr. Trump’s comments prompted an explosion of warnings about the dangers of any improvised remedies. Emergency management officials in Washington State posted a warning on Twitter. “Please don’t eat tide pods or inject yourself with any kind of disinfectant,” they wrote, before urging the public to rely only on official medical advice about Covid-19. “Just don’t make a bad situation worse.”

The maker of the disinfectants Lysol and Dettol also issued a statement on Friday warning against the improper use of their products. “As a global leader in health and hygiene products, we must be clear that under no circumstance should our disinfectant products be administered into the human body (through injection, ingestion or any other route),” the company said.

By Friday morning, the White House press secretary, Kayleigh McEnany, issued a statement: “President Trump has repeatedly said that Americans should consult with medical doctors regarding coronavirus treatment, a point that he emphasized again during yesterday’s briefing. Leave it to the media to irresponsibly take President Trump out of context and run with negative headlines.”