Wool fibers naturally drive off moisture, so this moisture does not stay in the fabric. This keeps the skin dry, but also prevents the formation of mildew or fungi. Wool also has natural antimicrobial properties, because bacteria tend to be attracted by smooth, positively charged surfaces of synthetic fibers, than by the scaly, neutrally charged surface of a wool fiber. Hospital studies indicate that bacterial colonies often occur in cotton sheets, but not in woolen blankets under the same conditions.