No one has screens on their windows or doors.
Screens, those metal frames with wire mesh that pop in place in your window or exist as a second door do not exist. Since no one owns an air conditioner and everyone’s house needs some ventilation various entrances to ones dwelling are often left open particularly in the summer months.
Therefore flies thinking this open door policy is the key to the apartment unceremoniously buzz around the German’s meticulously clean and ordered Haus. I did find a screen alternative in the supermarket that seems to be somewhat popular. You can buy a screen curtain, cut it to size, and hang it up by tacking Velcro to your window and door frames. From personal experience this is only partially effective but better than nothing.