The Centers for Disease Control and Prevention reported Thursday that the number of severe vaping illnesses has risen again, to a total of 2,506 cases nationwide.

That's an increase of 97 patients since last week — all hospitalized with EVALI, an acronym for e-cigarette or vaping product use associated lung injury.

There's no indication investigators are any closer to figuring out the exact cause of the illnesses, though there have been clues. Many cases have involved counterfeit weed pens. And vitamin E oil, a carrier oil used in some illicit vapes, has been found in the lungs of a number of patients.

The CDC has said there are likely multiple substances or products that are causing the fatigue, coughing, fever, trouble breathing, vomiting and diarrhea associated with EVALI.

Fifty-four people in 27 states and Washington, D.C., have died from the illness, and more deaths are under investigation.

Though the outbreak of vaping lung illnesses escalated rapidly in 2019, vaping in general has been steadily rising in popularity for years, mostly among young people.

A report published Thursday in the American Journal of Public Health found that a growing number of youth are experimenting with e-cigarettes at younger ages.

The percentage of children who began using e-cigarettes by age 14 has tripled since 2014, from 9 percent in 2014, to 29 percent last year, the report said.

It's a troubling finding, as addiction experts say nicotine is particularly harmful to young brains still in development.

"We know that kids that start using tobacco and nicotine products at earlier ages are more likely to become addicted and to keep using them in adulthood," said the study's author, Rebecca Evans-Polce, an assistant research scientist at the Center for the Study of Drugs, Alcohol, Smoking and Health at the University of Michigan.

The report focused on data from the National Youth Tobacco Survey, an annual government report on tobacco trends among students in middle and high schools across the country. Survey responses from more than 26,000 students were included in the new study.

Evans-Polce told NBC News that parents and doctors need to be aware that children as young as 12 are experimenting with vaping. Her research did not specify what substance they were vaping.

The new findings come on the heels of a separate report published this week that found the number of teens vaping marijuana has doubled since last year. From 2018 to 2019, the percentage of high school seniors who reported vaping pot within the past month rose from 7.5 percent to 14 percent.

Overwhelmingly, teenagers and people in their 20s are not vaping to quit regular cigarettes, despite some anecdotes from longtime adult smokers.

The American Academy of Family Physicians surveyed 1,000 people from 16 to 30, all of whom said they vaped tobacco products.

Fifty-nine percent said they did so "to relax." And survey respondents who were teenagers or in their early 20s were more likely than the older crowd to say they vaped to socialize or fit in with their peers.

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