Regardless of which strain of infl uenza virus causes the next pandemic, it will be highly transmissible between humans. Transmission of infl uenza is primarily airborne, through aerosolized respiratory tract secretions expelled during coughing and sneezing, although transmission by direct and possibly indirect contact may occur. Transmission can be expected to occur in various settings, including homes, healthcare facilities, schools, work sites, public transportation, and other settings at which people gather for social, commercial, or entertainment purposes. Higher exposure risk among particular population groups as a result of factors such as crowding and occupation could contribute to health disparities among socioeconomic and racial/ ethnic groups during an infl uenza pandemic.