Most people know to wear sunscreen to avoid sunburn and to protect against skin cancer if they're planning to be in the sun for any extended period of time. But can you imagine being so sensitive to ultraviolet (UV) light that you have to avoid sunlight through windows or even florescent lights?
Such is life for those who have xeroderma pigmentosum (XP), which happens to be about only 1 in every 250,000 people in the United States [source: XPmutations.org]. The dangers of sunlight to their skin are the same as for the rest of us -- that is, damage from UV light -- except that their skin is extremely sensitive to it.
Symptoms include not only sunburn and blistering (after little sun exposure), but also light and dark skin patches. Skin can also become so thin that blood spots and vessels are visible [source: New Zealand Dermatological Society]. As you might have guessed, such sensitivity almost inevitably leads to the development of skin cancer.
As a genetic disorder, XP occurs as a result of a faulty DNA repair system. Most people's bodies can repair DNA damage caused by sun exposure, but those with XP cannot. Although doctors can treat the symptoms, there is no known cure. It is so bad that children growing up with XP usually develop skin cancer and die by the time they become young adults [source: NIH].