In addition to being painful, badly sunburned skin can blister, peel, split and become infected. There goes your protective envelope, opening the door to invasive germs.
The solution: Cover as much of your body as possible. Long pants and full-coverage tops with long sleeves are best. A broad-brimmed hat, or a hat with with a Foreign Legion-style neck shade, will help. Keep your feet covered as well, wearing sandals can lead to burned feet, which will leave you unable to hike. Apply sunblock frequently--the higher the SPF rating, the better (see sidebar). This is especially important at high elevations and when you're near snow or water, because those conditions promote speedy and severe sunburn.