Horseflies are riddled with sooo much bacteria, im a little stunned that people are not seeking medical advice??
Listen to dunanddusted who has detailed through experience the severity.
When you are bitten, it is your bodies normal response to attempt to "get rid" of the invasion through the inflammatory response. No matter how old or young you are, a bite can be potentially life threatening (not to scare monger).
Signs or infection:
Reddness (bodies inflammatory response to fight infection)
Heat (feel the area, does it feel hot?)
Pain (everyones different here, some may feel pain, some not)
Itching (this is the release of histamine on your body to attempt to assist in stopping infection - a sign that your body is reacting to foreign substance)
Swelling with or without weeping from bite area
If the swelling spreads, and continues to be hot to touch - you need to get to gp for antibiotics.
Best way of seeing if it has spread and monitor it is to draw a line around area. if it spreads beyond line, seek gp/medical help.