At least 127 people in 15 states have fallen seriously ill with lung damage and difficulty breathing, confounding doctors struggling to diagnose and treat these patients.
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.
NBC News reached out to state health departments and physicians nationwide, and found that cases of the mystery illness now stretch from coast to coast.
California is looking into 19 possible cases, New York has 13 cases, New Jersey 9, and North Carolina and Virginia each have a handful of patients.
Still, most of the cases are clustered in the Midwest, with 35 confirmed in Indiana, Illinois, Indiana and Wisconsin.
Those states also have 34 cases more under investigation. Other states with potential cases include Connecticut, Iowa, Michigan, Ohio, Texas and Utah.
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 100-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."