After eight other otherwise, healthy teens were admitted to Children’s Wisconsin with with serious respiratory problems, doctors grew alarmed, reports the Milwaukee Business News.

Their symptoms included fatigue, shortness of breath, a cough, and chest pain. The influx of patients presenting with these problems, lead the Children’s staff to try to determine the mysterious cause. “We weren’t sure what we were dealing with,” said Dr. Michael Gutzeit, chief medical officer at Children’s Wisconsin.

We were concerned it was a public health issue because of the intense cluster of patients, he said. The staff thought it could be an infection of some sort that was spreading through the community.

“There wasn’t anything we could put together that would explain why these patients were so sick and why they were having such breathing difficulties,” Gutzeit said. “The thought process was: something’s irritating these lungs and it’s something that happened relatively abruptly, because they were healthy before this. Thinking through that, what would they all have been doing at the same time that would be the common thread here? And that’s what led to vaping. It was the only common thread we could see that would explain the symptoms that were happening.”

Read more about how Children’s Wisconsin handled this health crisis here.