A whopping number of people still use tanning beds and about one in three U.S. adults say they’ve used one before, (according to 2014 data).

Dermatologist Dawn Marie Davis, MD, an associate professor of dermatology and pediatrics at the Mayo Clinic, tells Yahoo Health that bacteria and virus can survive in tanning beds, despite the heat.

“We all have bacteria on the skin, which you’ve transferred to the bed. Then it’s being heated, but not so much to kill [the bacteria], then it becomes stronger and hardier,” Davis says. “Then if you sweat, that only adds to the bacteria or virus’s ability to grow, and then if you have a nick or cut in your skin, which is very common, then you’re much more likely to acquire the infection.”

Davis says staphylococcus bacteria (which can cause skin infections), fecal bacteria, herpes virus, and wart-causing human papillomavirus are able to survive in a tanning bed. “In my practice, I’ve seen acquired bacterial infections, warts, and herpes infections from tanning beds based on the timing of the infection and distribution of the infection,” she says.

Many people think the ultraviolet light is able to kill off any and all germs lurking in the tanning bed. They also assume that all the technicians in a salon thoroughly clean the tanning beds after each use, and that the person who used the tanning bed right before wore clothing, like a swimsuit, while they tanned.

But the truth is, all of these factors aren’t always the case — “and when you put that all together, ultraviolet can theoretically kill germs, but it’s not enough to kill germs on the tanning bed. Bacteria and viruses can grow and replicate on the tanning bed,” Davis says. “What’s interesting is that when you use the ultraviolet light over and over again — and the tanning bed lights are on for multiple hours a day — if the bacteria or virus is exposed to some antiseptic but not enough to kill it, or some light but not enough to kill it, it can grow stronger and then it becomes resistant.”