Flies!!!

Ok this is going to be a stupid post but I have to vent...
What is with all the stupid flies and how do they get in my house???? I just killed about 10 flies that were on my bedroom window this morning. Then I go in there after being gone for a couple hours and there was 20! It's like plucking a gray hair, 2 grow in it's place. I thought the cold was supposed to kill them anyway! I have more than in the summer time. Plus I absolutely abhor flies! They are so gross and dirty! You'd think I'd be used to them, growing up on a farm. They love manure and we had a ton of it with all the animals around. I never got used to them. I can remember going outside where they used to congregate on our garage doors with a fly swatter and going crazy with it. I guess I thought that if I killed enough of them, they'd quit hanging out there. I don't think it ever worked but I sooooooo wanted to get rid of them that I kept trying. Well, I don't do that anymore but I still try to keep them out of my house. Ugh! They are gross!