The Scenario: Your friend isn't the cleanest or most hygienic person around. OK, fine, he's a slob. He hates doing laundry and sees nothing wrong with wearing the same clothes—namely, underwear—for too many days in a row. Not washing clothes as often saves water and thus the environment, he argues. And if the tighty-whities pass the sniff test—and there's no brown streaks—he thinks it's totally acceptable to wear them another day (or two, or three).
The Concern: As the barrier between your private parts and your clothing, underwear serves an important job of catching discharge or bodily fluid residue. In other words, there's some nasty shit—literally—accumulating down there. Let's also recall that bacteria thrives in warm, moist environments. Wear the same pair over and over, and it could only be a matter of time before some mysterious bacteria colonizes your crotch.
"Moisture in the groin area leads to overgrowth of yeast," says Lauren Eckert Ploch, a New Orleans-based dermatologist. This can cause several different types of rashes, including intertrigo (which usually turns up between folds of skin) and tine cruris (otherwise known as crotch itch). Bacteria can also overgrow: Many pathogens live on our skin normally, but can rapidly proliferate under the right conditions. Those "right conditions" are fueled largely by heat and moisture—a.k.a., the climate in your boxers or panties at any given moment. Ploch says the buildup of bacteria or yeast can also lead to similar itching, pain, and rashes. Too much bacteria, and you're probably looking at acne bumps or a cyst in the groin area.
The Worst That Can Happen: Aside from a gnarly rash, letting bacteria grow and multiply could lead to a pretty severe bacterial infection like staph, which is from the bacteria staphylococcus. A staph infection in your nether regions sounds painful, and it can get dangerous if not treated properly. In rare incidents, staph can be deadly if it's ignored, although it's usually cleared up with antibiotics.
What Will Probably Happen: With all that bacteria having a field day in your skivvies, you'll be really smelly. Like, radiating odor, no-one-wants-to-sit-next-to-you-on-the-subway-level rank. Ploch also warns that if women wear the same pair for several days in a row, a minor infection such as candida yeast is pretty likely to develop. Irritation is also a strong possibility, as is a rash or getting acne in the groin area. But you'll for sure stink; that's almost a guarantee.
What You Should Do: Tell your friend to change his damn underwear! Don't use the sniff test as an excuse. Ploch recommends wearing a new pair at least once a day, twice if you're super active and sweaty. So that pair your friend soaked during spin class in the morning should not be the same pair she wears the rest of the day. Got it? Saving your private parts from bacteria overgrowth is better than saving some water doing laundry. And if your friend is that concerned about the environment, he can hand wash them himself in cold water in the sink and hang them to dry. Instagramming his neo-Amish lifestyle is optional.