Horseflies, or cleggs, as they call them up north are right bastards. Being bitten once is an indignity, to be swarmed, as often happens, can result in bumps aplenty and in my case an allergic reaction. I was once bitten by a clegg in Scotland and my arm ballooned like something inflatable. The pus seeped for days. In the Pyrenees, walking the GR10 I was mobbed for what seemed like weeks by the nasty creatures. Multiple bites inflicted on bare legs started to get infected and finally I was bitten on the tongue- gasping for air on a steep ascent one flew in and got me. I had visions of my palate swelling and blocking all airways, my tongue expanding like the swollen licker of a hanged man- but no- nothing happened. Nothing. Saliva, as reported in 2008 in New Scientist, is a great natural anti-biotic. After this, as soon as I got bitten I now gobbed a big load of saliva onto the bite site (and killed the clegg if I could) and this stopped all further swelling. As long as I got the spit on the bite within 30 seconds I was fine. Not even a pimple. Later I found this works with all insect bites and is a great first treatment for any outdoor cut. Suck on it!
In the summer this post gets a lot of hits- presumably by people who have just been bitten. If that has happened, and you have an allergic reaction you may get a lot of swelling and it's too late to apply the 30 second-suck treatment. However you can keep rubbing saliva on the bite to stop it getting infected. You'll have redness, maybe some irritation and swelling but you'll be fine. If you do get an infection with painful lymph glands and a huge swelling go to the doctor for some antibiotics in the worst case. Moderate swelling will go down after a few days.