This paper reviews the results of investigations for animals which have been infected by instillation or spraying a virus suspension into the nostrils or by exposure to affected animals through a mask or by indirect contact. The results indicate that pigs are not readily infected by the intranasal route. Also, the nature of the aerosol droplets needs to be analyzed to determine how the respective amounts of infective and non-infective virus particles, host components and the presence of antibody affect the survival in air and the ability to infect the respiratory tract.