Human skin is typically a bit on the acidic side, making it an inhospitable environment for many types of bacteria -- when it's dry. When it gets wet, that's another story. Bacteria grow quickly on your skin when you sweat, but they start to make themselves at home if you're just moist. Warmth helps them get established, too. Moist and warm; they love that. In short, the conditions when you get out of the shower are perfect for a bacterial bloom unless you use soap and dry off thoroughly with your trusty, fluffy towel.