Illustration of antibodies (pale orange) attacking a foot-and-mouth disease virus (FMDV, pink and purple) particle. Antibodies are protective proteins produced by the immune system in response to foreign substances such as bacteria and viruses. FMDV is a highly contagious virus that infects cattle, pigs, and sheep and many cloven-hoofed wildlife species. It causes painful lesions to appear in the mouth and feet which may lead to loss of appetite and lameness. An outbreak of foot and mouth disease in the UK in 2001 caused the loss of over half a million cattle and three million sheep.