9. Sunlight (Darkseekers)
In the post-apocalyptic movie I Am Legend, an outbreak of a condition known as the Krippin virus which was created by genetically altering the Measles virus in a failed attempt to cure cancer results in some of Earth's population turning in to primal, aggressive cannibals called Darkseekers. The Darkseekers maintain only a modicum of their human intelligence and seem to be much stronger than the average human, while also seemingly having a massive pain tolerance. They lose their hair and their skin tone changes to be much paler - making them look incredibly creepy whilst also being a massive physical threat. However, they suffer from a huge sensitivity to sunlight - hence their name - and it can in fact be fatal to them if they're exposed to it for more than a second or two.