I would go for an infectious agent, a bacteria or a bigger parasit, that increases agressivity and metabolism, but at the same time depletes (consumes) some hormone or protein commonly found in blood.
That way, you get strong and fast people with a craving for bllod.
Theyy don't really need light to cause them physical harm. They can just be highly sensitive to strong light, which is an existing conditions, and to other stimuli, like the smell of garlic.
If you really want light to cause them harm, you could have the parasit screw up with pigmentation or repair mechanisms of the skin, making them extremely prone to sunburns.
If there is no answer, there is no question. If there is no solution, there is no problem.
Waffles to space = 100% pure WIN.