Gross Problem- I need help...

I've never had a housefly problem in Las Vegas before. Seriously not even enough to purchase a flyswatter. This summer hasn't been that bad- a few more then normal. But the last week and a half? It's out of control. I'm killing between 15-30 flies a day in my house! That's insane and disgusting. What the freak is going on and how do I stop it!!??