The most important infectious disease of pigs is salmonellosis, caused mainly by Salmonella choleraesuis and occasionally by S. typhi-murium. It is found wherever pigs are kept and may affect all members of a herd. Acute, subacute, and chronic forms of the disease are recognised. It is the main cause of pigs' deaths and failure to thrive. Necrotic ulceration of accidental or surgical wounds, caused by a mixed spirochaete and fusiform bacillus infection, is frequent and may affect entire herds. Ulceration is progressive and, if untreated, may lead to death. Localised in the digits it causes footrot and, in the spermatic cord (after castration), scirrhous cord. Mild forms of Inclusion body rhinitis are often found in young pigs and have marked effect on their thrift. The only important internal parasite. Hyostrongylus rubidus is found in all pig-rearing districts and infects pigs of all ages. Ascaris lumbricoides, Stephanurus dentatus, Trichuris trichura, Oesophagostomum dentatum, and Macracantho-rhynchus hirudinaceus are less significant parasites. External mange due to Sarcoptes scabiei, and infestation with the pig louse, Haematopinus suis, are common. Both cause loss of condition.