Deer flies are the worst, and there isn't a spray available that keeps them away. If you don't know what they are, they look like a big fly with 'V' wings, and they like orbiting your head, then darting in and biting you. They love trying to bite me around my eyes, and the bites are like large mosquito bites and take a week to heal.
But if it was end of the world, I'd just smear mud on my exposed skin. You'd be amazed how well that works.
Or they'll trigger a reaction or carry a disease, and it'll turn into a large, feverish rash.
Them and horse flies.
It's not just a "bite" either - they slice into you and lick up the blood. Little assholes.
Fun to see: Skin-So-Soft skin oil was once recommended to me as a bug repellent. It didn't seem to help in that respect, but I did somehow find an interesting effect: Get a large black pan, or a nice big impression in black asphalt, and put some of the oil into it so as to create a film.
I don't know the reason, but the deer flies like to fly into puddles like that. But then the oil coats their wings, and soon you'll have hundreds of drowned flies. It needs to be cleaned out periodically because the layer of dead flies will allow any newcomers to survive.
Even just walking out to get the mail in summer sucked back at my parents' house. No more than 10 feet from the front door and a fly would start orbiting your head, prepping to land and slice right in.