The verdict: No. Oil-based products are generally lower risk than water-based products, but higher risk than powders because they're usually applied on susceptible and sensitive areas like your eyelids and lips. If you like to live dangerously, Hammer says you can sharpen eyeliners and lip liners or hack off the top of lipsticks in an attempt to get rid of some surface bacteria, but it's still a gamble. There's no way to tell how far a virus has traveled.

"Viruses can't survive long enough outside the body to be transmitted through makeup that's been mailed and then properly sanitized. Using rubbing alcohol (of at least 70% volume or higher) and a spray bottle you can gently spray the top of your makeup and it should kill at least 99% of germs!" - Devin on Facebook

Experts say: I asked Hammer to watch the video posted on Makeup Exchange that instructs users on sanitizing makeup with alcohol before they ship it off to a new home, a practice many Facebook users also swore by. I was still skeptical -- it can't be that easy, can it? "There's actually some good information in this video, and some of these tips will help keep makeup products relatively germ-free," he cautiously began. But ... (you knew there was a but). "But sanitizing makeup with alcohol will generally only treat the surface, and may not be able to reach bacteria that's been pushed deeper into the product," he says.

The verdict: No. As with the whole general concept of makeup exchange, Hammer says there are just way too many unknowns to rely on this method. "To me, it's a bit like finding a tasty-looking, half-eaten sandwich on the ground, and then cutting off the part where someone else was eating it, wiping off the rest of it, and then hoping that it will be safe to eat. But how long was it on the ground? Did anything unusual happen to it while it was there? Maybe the person who was eating it had the flu?" Touché, Mr. Hammer. Touché.

"No, I wouldn't swap used makeup. [I'm] afraid of eye infections (which I got at a BareMinerals store) or getting a cold sore, which I've never had before." - Karen on Facebook

Experts say: OK, so you didn't heed expert advice and woke up red and swollen. How can you really tell if it's your grody, infected eyeliner or merely a bad reaction to that particular brand of makeup? "An allergic reaction will usually lead to redness, hives and possible swelling," says Ostad. "An infection or anything related to bacteria on the other hand may lead to sores, red spots, blisters and massive swelling on the face."

The verdict: Differentiating what kind of disfigurement or pain you're experiencing can help you track down the culprit, but Hammer warns that sometimes, we don't even need a second party to infect ourselves. "Although products are commonly formulated and tested using effective preservative systems, many different types of bacteria can live in cosmetic products, particularly as the product ages, or is used repeatedly."

The most common include the previously mentioned pseudomonas, staph infections and coliform varieties, as well as many different types of yeast and mold. So, vigorously pumping your mascara wand or leaving your eyeliner cap off as you set it back down on your bathroom counter (guilty) also falls into that "should I be putting this all over my face?" litmus test. And it's safest to assume your fellow swapper is sabotaging their makeup in the same manner.