Glad you asked...
I always called deer flies horse flies. I'd get bitten while riding on a horse. Kill the fly first. Use warm water to wash where it bite you at and you can pour some alcohol or peroxide on the bite.
Then again, this answer might help you...
First move away from the area that you were bitten at to treat deer fly bites. Wash the infected area with some warm soapy water. Ice will reduce the pain and swelling, so apply if necessary. Then rub a hydrocortisone cream on it.

