Hygiene can be, well, ambiguous. While there's no debate on whether it's okay to pee in the pool—the fact is, it's just plain gross—many of us choose to neglect the unwritten rules of hygiene, and heighten other people's risk of contracting unwanted germs, diseases, and illnesses. 

The flipside of that, however, is that many people are unknowingly engaging in risky activities, doing seemingly normal things that are actually far nastier than they think. We compiled a list of things we thought were surprisingly disgusting, and ran them past our experts to weigh in. But honestly, just...stop. 

Blowing the candles out on a birthday cake

Now that someone has said it, you can already envision it happening. The overjoyed young boy on his tenth birthday, wishing for a new Xbox, closes his eyes and takes a deep breath in, ready to put out the candle fire with a mixture of youthful wind power and saliva. "Kids are virtual petri dishes of everything," says Donna Duberg, an assistant professor of Clinical Laboratory Science from Saint Louis University. 

According to a study done by Clemson University, significant increases in bacteria were found in the icing on birthday cakes after the candles were blown out. Given the fact that this tradition has been in existence for decades, it's tough to gauge the threat of these bacteria. That said, it's probably not a smart idea for a sick kid to be spitting all over cake for the entire party. 

"Kids are typically more likely to have colds and flus," says Charles Gerba, Ph.D., a professor of Microbiology and Environmental Sciences at the University of Arizona. 

Bringing your coffee into the bathroom

You've just left the break room with a fresh cup of joe when an unexpected urge causes you to take a detour to the bathroom. What's wrong with just popping that on the sink while you take care of business? 

"Office coffee cups are loaded with fecal matter," says Gerba. "Because they're cleaned with sponges, it's easy for fecal bacteria to grow in large numbers." Think twice before you bring your beverage into the can. 

Sharing your headphones

Earwax is the most obvious grossness factor when sharing headphones and earbuds, but according to Duberg it should be the least of your concerns. 

"You're worried about earwax, but that doesn't grow anything. It's the ear canal that grows stuff," she says. 

She's more concerned with spreading bacteria like pseudomonas aeruginosa, which give off an odor that resembles sweet grapes, or causing otitis media (ear infections), a common risk for children who contract streptococci as a result of frequent sharing of headphones and earbuds. 

At the end of the day, that coat of visible earwax may seem like the grossest part, but the true danger lurks deeper. Keep your headphones to yourself. 

Bellying up to the buffet

There are plenty of good things about buffets: unlimited food, tons of variety, and, well, that's it. Unfortunately—as much as you might be comforted by that sneeze shield—buffets can be major breeding grounds for food-borne illnesses. 

There have been numerous incidents where buffets have cultivated diseases—E.coli, Salmonella, and Shigella are all potential risks when standing in line. Duberg recognizes the potential threat buffets can pose, and urges caution. 

Bobbing for apples

Your vision of this classic Halloween party game is now forever warped with the thought of blinded contestants hopelessly shoving their open mouths into an opaque pool of murky apple water. All that saliva swirling about in that pot is a recipe for disaster. 

So what's the solution to avoiding contamination from this classic party game? "There are alternatives where people are using chopsticks instead of their mouths to pull them out," says Duberg. Bob at your own risk, but it seems pretty wise to avoid shoving your head into a pool of everybody's spit.

BDST: 1559 HRS, MAY-28, 2014