Take, for example, soap. It's the definition of clean, right? Not so: Bars of soap often don't dry all the way, which means that bacteria and fungi can grow and be passed from person to person. Another major culprit? Manicure and pedicure tools — if they haven't been properly sanitized, they can transfer serious diseases such as hepatitis C. Same thing goes for razors and any other beauty tools (including toothbrushes!) that can come be exposed to broken skin — if they come into contact with your blood, it can spread an infection.
The one category of beauty products that probably is okay, according to dermatologist Neal Schultz? Makeup. As long as you or your friend don't have visible infections such as pink eye or cold sores, sharing makeup between friends is usually a safe bet, especially since many commercial makeup products include preservatives these days. One situation in which preservatives are actually a good thing, huh? (The Huffington Post)