The Centers for Disease Control and Prevention reported Wednesday that there are 149 possible cases of severe lung disease linked to vaping in 15 states: California, Connecticut, Illinois, Indiana, Iowa, Minnesota, Michigan, North Carolina, New Jersey, New Mexico, New York, Pennsylvania, Texas, Utah and Wisconsin.

The only thing linking the cases is that the patients all reported using vaping products that contain either nicotine or THC, the psychoactive ingredient in marijuana.

In addition to the cases reported by the CDC, NBC News reached out to state health departments and physicians nationwide, and found cases in Ohio, Virginia and Tennessee.

"One patient came in with full respiratory collapse and essentially had to be on life support," said Dr. Jacob Kaslow, a pediatric pulmonary fellow at Monroe Carell Jr. Children’s Hospital at Vanderbilt in Nashville.

Kaslow told NBC News that his hospital has treated four cases of vaping-related respiratory illness over the past six months. Tracking the cases has been tricky, as patients tend to have a variety of symptoms, including severe pneumonia and coughing up blood.

"We’re only discovering this now because we’re asking, 'is there any history of vaping or electronic cigarette use?'" said Kaslow.

No deaths have been reported. But some patients have developed severe, progressive lung disease, and have required ongoing mechanical breathing assistance.

The Centers for Disease Control and Prevention is working with states to try to pinpoint an e-cigarette ingredient, e-liquid, device or purchase method linking all of the cases.

It's unclear whether there was some kind of contamination of the devices or e-liquids that led to the 149-plus cases.

Some patients reported buying their vapes off the street.

"The evidence continues to point to street-bought vaping cartridges containing THC or synthetic drugs as being the cause of these illnesses," Gregory Conley, president of the American Vaping Association, wrote in a statement to NBC News.

The American Vaping Association is not a trade group, but does advocate for what Conley calls "sensible regulation" of vaping products.

Patients have typically arrived in the emergency department or hospital with cough, shortness of breath, fatigue and chest pain. The illness initially looks like a bad respiratory infection, but does not get better with usual treatment, like antibiotics.

This can delay a diagnosis and lead disease detectives down the wrong path.

Further muddling the investigation is that hospitals don't have a good way of tracking cases, because there is no specific diagnostic code for either vaping or the emerging disease.

"Since this is so new, physicians may miss the association with vaping," said Dr. Humberto Choi, a pulmonologist and critical care specialist at the Cleveland Clinic.

Choi has been treating some of the patients in the hospital's intensive care unit in recent days and weeks.

"Suspected cases should be reported to the CDC, but at this point, it’s difficult to track," Choi said. "As more cases are confirmed, we will be better able to characterize the disease and determine the best treatment for patients."

Choi has also seen people with similar but less severe disease in an outpatient setting.

"People with a history of recent vaping are coming in with abnormal CT scans," Choi told NBC News. "The only treatment is just to stop vaping."

CORRECTION (Aug. 23, 2019, 10:15 a.m. ET): A previous version of this article included Florida on the list of states with possible cases of vaping-linked lung disease reported to the CDC. After the article was published, the CDC removed Florida from the list. The article also stated that there were 153 possible cases reported to the CDC; with Florida off the list, the number is now 149.

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