Physics: The process of evaporation requires heat. That heat, in the case of a wet head drying over time, comes from your internal body temperature. If you allow your hair to dry over time, that chills you, and the drop in your body temperature leaves you more vulnerable to bacteria and viruses already in your system, increasing your chances of getting sick. But it all depends on the conditions of the room, and your own personal metabolism and immune system. This means not everybody catches colds from a wet head, but anybody -can-, depending on the conditions, so drying your hair with a towel and hair dryer is healthier.