Plastic window screen, also known as plastic insect screen,plastic mosquito screen ,nylon window screen or polyethylene window screen, is designed to cover the opening of a window. The mesh is usually made of plastic and polyethylene 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.