In the forbidden forest, there's these fairies that are almost like mosquitoes but instead of sucking blood, they suck magic and make their victim dizzy and feverish. When you get bit by one, the area becomes inflamed and you can see your veins. Most people (especially children and ill adults) die from the bite because the saliva of the Fairies is highly toxic and getting it into your bloodstream may kill you, if you don't die then it may just have side effects.