Dr. Pirzada is one of the many physicians across the country treating patients — now totaling more than 215 — with mysterious and life-threatening vaping-related illnesses this summer. The outbreak is “becoming an epidemic,” she said. “Something is very wrong.”

Patients, mostly otherwise healthy and in their late teens and 20s, are showing up with severe shortness of breath, often after suffering for several days with vomiting, fever and fatigue. Some have wound up in the intensive care unit or on a ventilator for weeks. Treatment has been complicated by patients’ lack of knowledge — and sometimes outright denial — about the actual substances they might have used or inhaled.

Health investigators are now trying to determine whether a particular toxin or substance has sneaked into the supply of vaping products, whether some people reused cartridges containing contaminants, or whether the risk stems from a broader behavior, like heavy e-cigarette use, vaping marijuana or a combination.

On Friday, the Centers for Disease Control and Prevention issued a warning to teenagers and other consumers, telling them to stop buying bootleg and street cannabis and e-cigarette products, and to stop modifying devices to vape adulterated substances.

[Read more: The Trump administration is weighing a ban on some flavored e-cigarettes.]

The illnesses have focused attention on a trend that has been overshadowed by the intense public concern about soaring teenage use of e-cigarettes, with its potential for hooking a new generation on nicotine: the rise of the vaping device itself. It has introduced a wholesale change in how people consume nicotine or marijuana, by inhaling vaporized ingredients.