Causes of listeriosis

Listeriosis is caused by a type of bacteria called listeria. It's mainly spread through contaminated food.

Listeria is widespread throughout the environment and can be found in soil, wood, decaying vegetation and water.

Contaminated food

Most cases of listeriosis are caused by eating food contaminated with listeria. Listeria is most commonly found in unpasteurised milk and dairy products made from unpasteurised milk.

Listeriacan also befound in food manufacturing environmentsandcan contaminate food products after production. For example, contamination can occur:

• after the food is cooked, but before it's packaged
• when food is handled in shops, such as on slicing machines or delicatessen counters
• in the home

Vegetables can be contaminated if they're grown in contaminated soil or fertiliser, or if they're washed in contaminated water. Meat and dairy products can become contaminated if they're taken from infected animals.

Unlike most other types of bacteria, listeria can survive and often multiply in temperatures below 5C (41F). Therefore, listeria can still grow to potentially harmful levels in foodstored in a fridge.

Read about preventing listeriosis .

Infected stools

It's thought that listeria can be found in the digestive systems of many animals, such as sheep and cattle, and these animals may pass stools contaminated with listeria.

It's estimated that up to 1 in 20 people may be carriers of listeria, but have no symptoms of listeriosis. Human carriers can also pass stools contaminated with listeria, which can spread if, for example, the carrier doesn't wash their hands after going to the toilet, then handles food.

At-risk groups

Some people are at an increased risk of developing listeriosis, including:

• those over 65 years of age
• pregnant women and their unborn babies
• babies less than one month old
• people with a weakened immune system, such as those with HIV and AIDS orreceiving chemotherapy

Pregnant women should avoid close contact with farm animals that are giving birth or have recently given birth..