I was just riding along a slightly overgrown byway, minding my own business (FS if that makes a difference) when I swallowed a fly.
What should I do? Will I die?

Maybe try finding a spider?
If the spider doesn't work, you may need to try a bird.
But that's absurd. Get a cat.
You will die
Some day.
For now? I hope your cat aids boosters are up to date.
You didn't vomit it up immediately when it hit the back of your throat? That's what's happened when I've experienced similar.
Anyway, I'm only a nurse so unable to offer medical advice, sorry 🤷♂️
Unlike Niall above I was a surgeon (admittedly of the arboreal type) .
We need to know the exact species to determine the effects. If you can sift through a stool sample and provide a positive ID of said fly we can then decide what type of cat aids you're going to get.
I had one once which tasted of aniseed. I can't stand aniseed.
The worst though was a daddy longlegs which wriggled at the back of my throat and made me throw up.
Not a doctor, so this is not medical advice, but take it from me; pass on the horse.
The need now is to sterilise your insides.
An evening on some single malt or vodka should work.
I had similar on Monday night, just cresting one of the few jumps on the local trail and a fly went into my open mouth and tried its best to get down into my lungs. Coughed a lot and have no idea if it came back out (never saw it if it did) or if it just got sucked down into a nice quiet corner of my lungs to fester and decompose. Was nearly sick with the coughing but felt mostly fine about 15 minutes later and haven’t gotten any worse since.
I swallowed a bee, well actually I spat if I'm honest.
I know it was a bee because it left the sting in the very back of my throat. Tried to get to it with my leatherman but no chance. Couldn't seem to dislodge it with anything so had to go to a&e where I experienced the joy of forceps to the back of the throat which is an experience I'm not particularly wishing to repeat!
“Brown paper and vinegar” (Sharpe's rifles).
”I recommend…amputation” (Dr. Channard).
Ooh, I had a wasp fly in once on a windy day, managed to stop it with my tongue, which it them stung, so I bit it and spat it out.
Weirdest sounding customer at a Screwfix ever, I might also have been dribbling. I couldn't swallow or eat properly that night.
I swallowed a fly the other night, what a relief. That tubes set up for incoming dead animals. It’s flies in the wind pipe that i find horrific
I heard that medically, it was more about the genetics of any kids you may have?
EDIT: mis-read thread title
I've been thinking for a while about cutting up a sieve, then mounting it across nose/mouth (hooked on helmet straps) as a fly guard. Plenty of air coming through/around, but fine mesh on the direct path. Is there such a thing already?
Just use a COVID face mask. There's problem a large supply going cheap somewhere
You absorbed the fly. Fly fusion. Now watch the Fly 1986 movie.