A window screen, also known as insect screen or bug screen is designed to cover the opening of a window. The mesh is usually made of metal wire, fiberglass, or other synthetic fiber and stretched in a frame of wood or metal. It serves to keep leaves, debris, insects, birds, and other animals from entering a building or a screened structure such as a porch, without blocking fresh air-flow. Most houses in Australia, the United States and Canada and other parts of the world have screens on the window to prevent entry of disease carrying insects like mosquitoes and house flies. Formerly, screens were required to be replaced by glass storm windows in the winter, especially in regions like the northern United States and Canada, but now combination storm and screen windows are available, which allow glass and screen panels to slide up and down.