All replicate in the upper respiratory tract epithelium and occasionally in the gut. The most virulent eg distemper replicate in essential organs outside the respiratory tract, e.g. in lymphoid cells or neurones. This is linked to how readily their Fo and Ho precursor proteins are cleaved in different cells. Transfer is via faeces, if they replicate in the gut, or nasal aerosol/saliva, if they replicate in the respiratory tract.