Symptoms of the illness include coughing and breathing trouble that can become severe enough to require that patients be attached to ventilators. Some also have nausea, vomiting and fever. Many have vaped THC, the psychoactive ingredient in marijuana; some have vaped both THC products and nicotine. Some say they have vaped only nicotine. [Maps and charts tracking the outbreak of vaping illnesses.]

It is not yet possible to tell whether the illness comes on quickly or is the cumulative effect of long-time vaping.

The exact cause of the illness is still not known, although C.D.C. officials have been referring to “chemical exposure.” The culprit could be one or more ingredients in the vaping fluids, or a toxin released from the materials used to make vaping devices, which contain heated coils that vaporize fluids or other substances. Many of the ingredients in the products are unknown.

“I wish we had more answers regarding the specific harmful products or components that are causing these illnesses,” Dr. Schuchat said.

She added: “I think we have the feeling right now that there may be a lot of different nasty things in e-cigarettes or vaping products, and they may cause different harms in the lungs.”

In some cases, the injury to the lung tissue looks like a chemical burn, the same kind of damage that occurs from industrial accidents where chemicals spill and people inhale poisonous fumes, experts in lung pathology from the Mayo Clinic reported on Wednesday. Their findings were based on studying samples of lung tissue from 17 patients, ages 19 to 67, who became ill after vaping. Most reported vaping THC.

Medical investigators are scrambling to find the cause of the illnesses, a task made more difficult by the booming market in vaping products, some legal and many not, from sources unknown. Some are concocted at home by users themselves. All the products are a stew of chemicals, often including flavorings, oils and solvents that may react with one another when heated, to produce still more molecules that have yet to be identified.