Some patients have needed supplementary oxygen, including a ventilator in as many as a third of cases analyzed in The New England Journal of Medicine.

On lung scans, the illness looks like a bacterial or viral pneumonia that has attacked the lungs, but no infection has been found in testing.

What’s the best way to prevent the illness?

H ealth of fic ials say that the riskiest behavior is using vaping products bought on the street instead of from a retailer, or those that have been tampered w ith or mixed.

Mitch Zeller, director for the Center for Tobacco Products at the F.D.A., said, “If you’re thinking of purchasing one of these products off the street, out of the back of a car, out of a trunk, in an alley, or if you’re going to then go home and make modifications to the product yourself using something that you purchased from some third party or got from a friend, think twice.”

The C.D.C. and some state health officials have recommended that people give up vaping of any type until the cause of the lung damage is determined.

An editorial in The New England Journal of Medicine stated bluntly that doctors should discourage their patients from vaping.

E-cigarettes and other vaping devices were developed to help cigarette smokers quit their dangerous habit by providing a way to satisfy their nicotine addiction without inhaling the toxins the come from burning tobacco. But many medical experts now think even smokers should think twice about turning to e-cigarettes — and anyone who does not smoke should not vape.