Although airborne transmission of tuberculosis had been assumed since the late 19th century, studies to quantify the risk had been missing. In the 1950s, R. L. Riley et al. started a series of investigations on “air hygiene in tuberculosis.” In a cooperative study in Baltimore, Md., the authors exposed guinea pigs continuously for 2 years to air from a ward occupied by patients with active pulmonary tuberculosis, assuming that infectious doses for humans and guinea pigs were approximately the same. With painstaking accuracy and the help of the most recent methodologies available, the study was carried out to conclude that even small numbers of tubercle bacilli can account for the spread of human tuberculosis. Even typing with the help of susceptibility patterns (unfortunately, the technique is not described) was attempted to trace the infections in guinea pigs to a particular human source. This study revealed that patients with the highest counts of tubercle bacilli in their sputa were also the most infectious ones. This study and the companion ones were pivotal for subsequent air control measures recommended for rooms with tuberculous patients and have assumed new importance with the increase in tuberculosis in general and drug-resistant Mycobacterium tuberculosis strains in particular (cf. S. Segal-Maurer and G. E. Kalkut, Clin. Infect. Dis. 19:299–308, 1994). ALEXANDER VON GRAEVENITZ