Screens for windows and doors protect against germs and infection. We may think of them as old-fashioned, but they are still a first-line defense, as effective now as when they were first used in the 1860s.
For hundreds of years, Europeans and Americans kept their windows and doors closed at night, no matter how hot it was, because they were afraid of night air. It was believed that bad air rose from the ground at night and spread into houses, causing illness.
Two changes in medicine in the late 1800s led to open windows and widespread use of screens. First, medicine accepted the germ theory of disease (and rejected the bad air theory). Second, research began to show that many deadly diseases were transmitted by insects.
People then faced a conflict between the desire for fresh air and the need to keep out insects. The solution was “wire cloth,” already used in sieves for flour, cheese making, and other tasks. The wire mesh was put into frames to create window screens and later screened doors, and screened porches to keep out insects while letting air circulate.