Now young people are being sickened by the new wave of lung illnesses. C.D.C. officials said they believe that some “chemical” is involved as the cause but they have not identified a single responsible “device, product or substance,” Dr. Meaney-Delman said.

Dr. Christiani wrote in The New England Journal editorial that it was not yet clear which substances were causing the damage. E-cigarette fluids alone contain “at least six groups of potentially toxic compounds,” he wrote, but he noted that many of the patients had also vaped substances extracted from marijuana or hemp. The mixed-up stew of chemicals might even create new toxins, Dr. Christiani suggested. The journal also published a study of two clusters of 53 cases in Wisconsin and Illinois.

What looked like scattered cases earlier this summer has become a full-fledged and widespread public health scare, leaving otherwise healthy teenagers and young adults severely ill.

The first case of the mysterious lung illness, in Illinois, came in April , indicating that the syndrome emerged earlier than the mid-June date that federal officials have often cited as the time the afflictions began.

The patients studied in Illinois and Wisconsin were typically “healthy, young, with a median age of 19 years and a majority have been men,” said Dr. Jennifer Layden, chief medical officer and state epidemiologist for the Illinois Department of Public Health. A third were younger than 18.

The journal article about the Illinois and Wisconsin patients said that 98 percent were hospitalized, half required admission to the intensive care unit, and a third had so much trouble breathing that they needed to be placed on ventilators.

Eighty-four percent had vaped a product including T.H.C., the high-inducing chemical in marijuana . Dr. Layden said a majority had also used a “nicotine-based product,” noting that there were “a range of products and devices.”