SIOUX FALLS, S.D. — If you're a patient at a Sanford Health facility and you smoke e-cigarettes, your doctor has something to tell you: stop.

Sanford's chief medical officer, Dr. Allison Suttle, sent a directive to all Sanford physicians Wednesday afternoon, calling on them to help curb a nationwide outbreak of a severe lung illness linked to use of e-cigarettes, or the practice of vaping. The outbreak has sickened more than 500 and killed seven, the Centers for Disease Control said Wednesday, Sept. 18.

"I am asking you to reach out to these patients as you see fit to warn them of the potential dangers of e-cigarettes," Suttle said to Sanford physicians in the email.

Over the next two weeks, the health system will send a letter to all patients with a history of e-cigarette use telling them to stop and talk to their doctor and state quit line about halting the practice, Suttle said. Sanford also created a quick reference and call-to-action for document for physicians to review.

Officials with the CDC said this week there have been 530 reported cases of the disease nationwide with seven confirmed deaths, and they expect more to be reported.

"The e-cigarette or vaping-related lung injuries we have been describing are serious," said Dr. Anne Schuchat, CDC principal deputy director, in a Thursday teleconference. "People are dying."

Sanford's call to quit e-cigarette use preceded a Thursday call by federal health officials where they again cautioned e-cigarette users to avoid the practice and said not to use or buy products off the street or modify or add any substances to their e-cigarettes.

Federal and state officials are still scrambling to understand what exactly is triggering the illness. As yet, they say there's not one single substance that appears to be involved in all cases. Many, but not all, patients stricken by the illness, have used products that contain THC, the key psychoactive ingredient in marijuana. Others have reported using e-cigarettes containing only nicotine.

"We are leaving no stone unturned in following all potential leads regarding any particular product, constituent or compound that may be at issue," said Mitch Zeller, director of the Food and Drug Administration's Center for Tobacco Products, in the Thursday media briefing.

The FDA's law enforcement arm, the Office of Criminal Investigations, has opened its own parallel investigation into the illness, including examining the supply chain of products used by patients, Zeller said.