I've killed at least a horde.

Flies keep appearing around the windows of my apartment. The closed windows of my apartment. They're driving me insane. I can't figure out where they're coming from. Unless they were bred right here in my apartment, which is gross. But why would they go from invisible to huge and slow? When I see one, I grab my flip flop and kill it. It's such an awful and disgusting process, however, that I never try to seek out more. But this morning I said, "enough is enough!" So I drew up all the blinds and killed about seven. It worked for a few hours. Early this evening, however, I killed another five. Seriously, where are they coming from?! Are they sneaking in through a crevice in the window? Or the side door, maybe that's it. But they're so huge, how do they do that? And it's not like they're hanging out around the kitchen or trash, so I don't feel like my apartment is this sty that attracts them. Anyway, they're gross and I'm about to go nuts.