You know that sharing makeup is a no-no, and for good reason.

“If a woman has a cut on her lip and borrows lipstick from someone who has a cold sore, she’ll get a cold sore. You can pass herpes [the cold sore virus], conjunctivitis [pink eye] and all sorts of things through sharing makeup,” says Dr. Zein Obagi, a dermatologist based in Beverly Hills.

Now imagine sharing your makeup with a few thousand or so of your closest friends when you sample makeup in testers in stores. If you don’t insist on practicing safe hygiene, or insist that the workers behind the counter practice safe hygiene when they’re applying makeup to your skin, it can get ugly fast.

Dr. Elizabeth Brooks, a biological sciences professor at Jefferson Medical College in Philadelphia, conducted a much-quoted two-year study on public makeup testers when she was with Rowan University in New Jersey about five years ago. “We went to department stores, specialty stores, drugstores — everywhere,” she says. Her researchers found staph, strep and even E. coli bacteria on makeup testers. “Wherever you see E. coli, you should just think ‘E. coli equals feces,’ ” Brooks says. “That means someone went to the bathroom, didn’t wash their hands and then stuck their fingers in that moisturizer.” Brooks says that when they tested the makeup on Saturdays — the day with the most traffic at cosmetic counters — the percentage of tainted makeup was 100%.

The FDA concurs. In recommendations for cosmetic safety, the agency warns against sharing makeup, saying, “Don’t share or swap eye cosmetics — not even with your best friend. Another person’s germs may be hazardous to you. The risk of contamination may be even greater with ‘testers’ at retail stores, where a number of people are using the same sample product.”

But there are solutions. For instance, if you want to try lotions, use samples that people don’t have to put their fingers into; instead of sampling from a jar, try a lotion you can squeeze on instead.

You can also clean the surface of the makeup tester with a tissue or a tissue dipped in alcohol before applying makeup. If you’re trying lipstick, debride it with a disposable applicator. And always use disposable applicators or cotton swabs, never communal makeup brushes. “If the brush is made of animal hair, bacteria can grow into the actual hair and be harder to clean,” Brooks says. You can also ask for a clean tester.

“Every makeup display that we tested, and we tested hundreds, always had disposable brushes and little pads to put on makeup, and they had all of that behind the counter but it was for the asking — they didn’t have it for the general public just to pick up at their leisure,” Brooks says. “And of course your Avon or Mary Kay lady had one-shot testers that have just enough to put on your lips or eyes, and some of the bigger brands had one-shot testers of foundation, lipstick or eye shadow as well — but not every brand carries that.”

Brooks’ most adamant advice is not to test publicly used makeup on your eyes, nose or mouth. “Mascara is the most troubling,” Brooks says. “I have two teenage daughters —I tell them never, ever, ever use a tester mascara because there could be cross-contamination, and this is your eye — there could be some serious ramifications.”

Brooks says a lipstick is usable if the surface layer is scraped off or if it is dipped in alcohol. “But if you’re asking me if I would personally try a public lipstick tester or if I would let my teenage daughters do that, the answer is no. I would ask for an individual tester,” she says. “Even when the ladies [behind the counter] are very diligent about dipping it in alcohol, they’re not lab technicians and I’m not 100% sure about it. Viruses are so small in comparison to bacteria it’s harder to get rid of a virus. Lipstick you can actually test on your hand and see if you like the color. I also wouldn’t test something like a face cream where I could not remove the top surface.”

But despite all of her research, Brooks is not a makeup killjoy. “Even doing all of this research, I’m not afraid or afraid to send my daughters to the mall. I just tell them not to put anything near their eyes, nose or mouth and you’ll be OK. I want women to be happy that they’re women and enjoy makeup but just be careful.”