Some people are at an increased risk of developing listeriosis, including: those over 60 years of age; pregnant women and their unborn babies; babies less than one month old and people with a weakened their immune system, such as those with HIV or on medication such as chemotherapy.
Listeriosis is caused by a type of bacteria called Listeria monocytogenes (listeria) and is mainly spread through contaminated food.
Listeria is widespread throughout the environment and can be found in soil, wood, decaying vegetation and water.
Unlike most other types of bacteria, listeria can survive and often multiply in temperatures below 5ºC (41ºF). Therefore, listeria can still grow to potentially harmful levels in food stored in a fridge.
Listeria cannot multiply in temperatures below 0ºC (32ºF), but freezing food doesn't necessarily kill all listeria bacteria.
Listeria can be removed by cooking food thoroughly or, in the case of dairy products, pasteurising it (a heat treatment designed to kill bacteria).