Although a few advanced thinkers understood the contagious nature of tuberculosis in the middle of the last century, more precise concepts of transmission awaited Pasteurs's demonstration in 1862 that organisms exist in the air and Koch's identification of the tubercle bacillus in 1882. At the end of the last century Cornet presented evidence that tuberculosis was dust-borne, and Flügge presented equally convincing evidence that tuberculosis was droplet-borne. Chapin, in 1910, summarized evidence against the prevaling belief in airborne transmission of many common infections but made an exception of tuberculosis. Chapin emphasized transmission by direct contact (touching). In the 1930's William F. Wells introduced the droplet nucleus hypothesis. In the 1950's droplet nuclei were shown to be the carriers of infection from tuberculosis patients to guinea pigs breathing air vented from the patients' ward. Subsequent evidence has indicated that direct contact is unimportant, although close proximity facilitates airborne transmission. Attempts to control the spread of tuberculosis by air disinfection or by immunization have been disappointing. Specific antituberculosis drugs, widely used since the 1950's, cause patients to become noninfectious for others very rapidly, even before organisms in the sputum disappear. Chemotherapy is now recognized as the corner stone of contagion control.