VIRUSES The viruses are little adapted to independent air dispersal. Some viruses infecting the animal respiratory tract are forced into the air on droplets during coughing and sneezing; but most bacterial and plant viruses, if they occur in the air at all, only get there on `rafts' of debris or water droplets. Some of the so-called polyhedral viruses infecting insects are exceptional. Reports of outbreaks among pests of forest trees in eastern Europe speak of copious yellow deposits of the polyhedral bodies, shed by parasitized insects, which coat the surfaces of vehicles travelling through the forests. Study of air dispersal of viruses might explain some of the anomalies in the behaviour of insect viruses. To prevent contamination by an airborne infective particle of the dimensions of a virus may well require quite unusual experimental precautions.