For more than a century, surgeons have been using masks to prevent microbes from their mouths and noses from falling into the open wounds of their patients. However, a 2001 paper in the Journal of Hospital Infection reviewed all known studies about "surgical face masks in the operating theatre" and found that their "effectiveness remains unresolved." Likewise, a 2016 paper conducted an extensive search for all available randomized controlled trials on surgical masks and found "there was no statistically significant difference in infection rates between the masked and unmasked group in any of the trials." Medical journals have published at least 20 studies and analyses showing that Covid-19 and other contagious respiratory diseases are mainly spread by microscopic aerosols. The vast bulk of these aerosols are less than 1 micron in diameter, which is 1/17th the size of the pores in the finest surgical masks, and less than 1/80th the size of the pores in the finest cloth masks.