Although food is not the only means of transmission, and in the majority of cases a food vehicle is not identified, listeriosis is considered to be a food borne illness. It is caused by Listeria monocytogene, which is widely distributed in the environment. It is commonly found in effluents and sewage sludge and survives many weeks after spraying. In one outbreak involving coleslaw, the cabbage, stored for several months, had been grown in a field fertilised by sheep manure. The bacteria may be excreted bu human or animal carriers, and many cases of cross-infection have been recorded. Symptoms are flu-like and include fever, diarrhoea, vomiting, septicaemia, meningitis and abortion; neonates, pregnant women, immuno-suppressed persons and the elderly are most at risk. The incubation period is 1 day to 3 months. It multiples between -1.5°C and 42°C and can multiply slowly in refrigerated foods (psychrophil).
A death rate of up to 30% is possible but mainly involves persons with other serious illness. The government had issued warnings to at-risk groups to avoid soft cheeses, cook-chill meals and pate, small numbers of listeria can be found in most foods from time to time.