When flies are buzzing around humans it's because the flies are attracted to to the heat of the warm body, to the body sweat and body salt, and the more you sweat the more attracted the flies are to you.
Flies are also attracted to the carbon dioxide that us humans breathe out.
I notice when I sweat more then the flies are more attracted to me.
So it's mostly the sweat and smell of the sweat and the salt that the flies are attracted too.
However some flies may land on my mouth or face.
Flies are pretty persistent annoying little things and even when you slap or swat at them without a fly swatter the flies will come back.
I sometimes even miss them with a fly swatter.
I hate flies and I actually have about 10 of them flying around in my living room right now.
I live near a cow feed lot that smells bad and even though the feed lot is around 10 miles away we get a lot of flies here and I can also smell the feed lot when the wind blows just right.