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Foot-and-mouth disease. Spraying disinfectant on a tractor wheel to prevent the spread of foot-and- mouth disease. This disease is caused by viruses in the picornavirus group. It infects domestic and wild cloven-hoofed animals, causing fever followed by the development of blisters in the mouth and on the feet. It is highly contagious, spreading by direct contact with infected animals and animal products, in the air and on equipment exposed to infection. Diseased areas may be quarantined to avoid further spread and disinfectant must be widely used. Infected farm animals are slaughtered and burnt to remove contamination. Photographed in Britain in 2001. (KEYSTONE/SCIENCE PHOTO LIBRARY/SIMON FRASER)