The early 2000s saw the surfacing of severe acute respiratory syndrome or SARS, the large-scale return of tuberculosis or TB, an influenza pandemic, and the intentional dispersion of diseases like anthrax – all of which are highly infectious airborne diseases. Authors noted that densely populated urban areas and offices, schools, other buildings, aircraft, and other mass transport vehicles (where people spent most of their time) were ideal for the transmission of SARS.