A method of preparing a viral aerosol from a dilute viral suspension prepared by dissolving a virus in an aqueous solution containing 6-12 g/l of a monovalent cation salt, or 50-100 g/l of a hexose, which is then nebulised with a gas pressure of 0.5-3.5 bars or an ultrasonic frequency of 2-5 MHZ. The resulting aerosol composition is also disclosed.