Climatic conditions are significant in the spread of airborne viral animal diseases. The virus is commonly attached to particles only a few microns in size, such as dust or dried saliva. These particles may be carried by the wind and then dispersed on the ground by rainfall. For this reason, diseases such as foot-and-mouth are of higher incidence during wet weather. Buildings act as a filter and trap for airborne material. Modern intensive animal rearing units therefore contribute to the risk of disease, especially with the use of mechanical ventilation which increases the throughput of air. Airborne viruses may travel distances of 30 or even 150 km. Animal fodder may be contaminated by infected airborne particles, but the importance of this in comparison with inhalation, depends on the dosage.