Most of the patients have recovered enough to go home after days or weeks in the hospital, but doctors say it is too soon to tell whether they will suffer permanent damage to their lungs.

The case count started to snowball in August after health officials made the connection between vaping and the lung problems, and reported it publicly. Doctors around the country then began to realize it might explain some of the odd, pneumonia-like illnesses they were seeing in otherwise healthy people, and many began to question their patients about vaping. The rising numbers include new cases but also earlier ones that are just now being recognized as doctors look back at past patients whose illnesses were not understood at the time.

Many people who became sick have said they were vaping THC, the ingredient in marijuana that induces a high, but some patients report vaping only nicotine. Some of the patients who died were older and had underlying illnesses, health officials have said. But on Wednesday, Mississippi health officials reported a vaping-related death of a person who was under 30.

No specific ingredients or devices have been identified as the cause of the illnesses, and health experts say multiple devices or ingredients may be involved. Investigations are being conducted by the C.D.C., Food and Drug Administration and state health departments.

Given the unanswered questions about the exact cause of the illnesses, many health experts say people simply should not vape. Those who continue to do so should not buy vaping liquids off the street, or add ingredients to commercial products, the C.D.C. said. It emphasized that adults who do not smoke should not start using e-cigarettes, and that young people and pregnant women should never use them.