Almost all patients with a mysterious vaping-related lung disease have been hospitalized, CDC says.

Officials are tentatively calling the illness “e-cigarette, or vaping, product use associated lung injury,” or EVALI.

At least 26 people have died from the disease and nearly 1,300 people have fallen ill in recent months

Almost all patients with a mysterious vaping-related lung disease have been hospitalized with about half ending up in the intensive care unit, Centers for Disease Control and Prevention officials said Friday.

At least 26 people have died from the disease that first came to light in July and nearly 1,300 people have fallen ill so far, the CDC said, adding that the spread of the illness shows no signs of abating. U.S. health officials are tentatively calling it EVALI, short for e-cigarette, or vaping, product use associated lung injury.

“I can’t stress enough the seriousness of these lung injury associated with the use of e-cigarette or vaping products,” Dr. Anne Schuchat, principal deputy director of the CDC, told reporters on a call Friday. “This is a critical issue and even while we learn more, we need to take steps to prevent additional cases.”

Among roughly 1,000 of the cases CDC has studied, 96% of the patients were hospitalized, the agency said. Of 342 of the cases, 47% of patients were admitted to the intensive care unit.

Doctors still don’t know what’s making people sick, and Schuchat said it may take several months to unravel the mystery. She said she thinks “there will be multiple causes and potentially more than one root cause. So I do think the phenomenon we’re seeing is going to have an explanation, but it may not be tomorrow.”

The Food and Drug Administration has received or collected more than 725 samples of the products patients were vaping from 23 states and the U.S. Virgin Islands, said Mitch Zeller, director of the FDA’s Center for Tobacco Products. The agency has started testing more than 300 samples, including 79 nicotine-containing products and 225 THC-containing products.

One possibility has emerged is Vitamin E acetate in oils containing THC, the compound that produces a high in marijuana. The FDA has found Vitamin E in 47% of the samples that contain THC. Adding Vitamin E to a THC cartridge can dilute the oil, possibly producing more vapor and allowing people to produce more oil for less money.

“This is an extraordinarily complicated investigation with a great diversity of products and intervening acts or actors that could be modifying these products along the way, especially for the great majority of these cases that involve THC and the presence of oils and other compounds,” Zeller said.

The CDC in its guidance urges doctors to ask patients whether they vape and what substances they used. Among nearly 600 patients, 76% reported using THC and 13% reported vaping only nicotine.

“We are not seeing a meaningful drop off in new cases and unfortunately many more people have been hospitalized each week,” Schuchat said.