Two Army soldiers have fallen ill with a severe lung illness linked to vaping, joining more than 1,500 cases either confirmed or under investigation across the country.

One of the soldiers is based in the U.S. and has since recovered, a spokeswoman for the Army Public Health Center told NBC News.

The other soldier is stationed overseas and remains hospitalized, the spokeswoman said.

Many of the severe lung injuries have been associated with THC vapes that may be counterfeit, especially ones that go by the name Dank Vapes. Some people, however, said they only vaped nicotine, not THC.

An Army spokesperson said the exact substances vaped by the soldiers are under investigation.

Last month, the Army, Air Force and Navy banned sales of electronic cigarettes from its stores on bases. The Army Public Health Center also issued a warning to its service members and their families discouraging the use of all e-cigarettes as health officials try to figure out what's causing the spate of vaping illnesses.

Tobacco use has long been ingrained in military culture, and veterans are more likely than the general population to smoke.

Nearly 30 percent of veterans reported tobacco use in a 2018 report published by the Centers for Disease Control and Prevention. That's more than double the percentage of U.S. adults who smoke.

But the rates of traditional tobacco use among current service members have been declining steadily in recent years. Vaping has largely replaced cigarette smoking, according to a 2015 survey commissioned by the Department of Defense.

The survey found 11 percent of troops said they used e-cigarettes daily, compared to 7 percent who said they smoked regular cigarettes every day.

"It should be considered a myth that vaping is less harmful than conventional cigarette smoking," Dr. Marc Williams, a toxicologist at the Army's health center, said in a statement.

"In efforts to conserve the fighting strength, and strengthen Army readiness and resilience, vaping of e-cigarettes should be highly discouraged at this time," he said.

Vaping illnesses continue to rise

The CDC has confirmed 1,080 cases of the severe lung illnesses linked to vaping, and will update its investigation on Thursday.

The number of those who've fallen ill after vaping is almost certain to rise.

State health departments tell NBC News they've either confirmed or are investigating 1,593 such cases. Alaska is the only state without reported cases.

On Wednesday, health officials in Georgia and in Utah confirmed vaping-related deaths in their states, bringing the national total to 26.

Health officials in Utah said the person who died was under the age of 30, and died at home without being hospitalized.

"This death is a sad reminder of the severity of these unexplained illnesses," said Dr. Angela Dunn, epidemiologist for the Utah Department of Health.

An investigation revealed the person had vaped a product containing THC, the psychoactive ingredient in marijuana.

"Based on what we know about this outbreak and what may be contributing to it, our best advice to the public is to stop vaping products that contain THC," Dunn said.

Across the country, patients all report a history of vaping either THC, nicotine or a combination of the two before developing an illness that resembles pneumonia.

In most cases, they arrive at the hospital struggling to breathe, and report severe cough, fever, fatigue, chest pain, vomiting and diarrhea.

Approximately 70 percent of patients are male, according to the CDC, and more than three quarters are under age 35.

Because investigators have been unable thus far to link all of the cases to any single ingredient, substance or brand, they continue to warn the public against all forms of vaping, including THC and nicotine.

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