The bacteria that cause listeriosis are widespread in the environment and can be found in soil, surface water, vegetation and a range of wild and domestic animals. Most humans catch the infection by eating contaminated food. The bacteria can grow at low temperatures and are tolerant of salt and therefore can survive in processed, preserved and refrigerated foods. Foods that have been associated with spreading listeriosis include the following: processed meat and fish, cold meats and hot dogs; dairy products, such as soft cheese, butter and milk, especially if unpasteurised; and pre-prepared salads, sandwiches and salads. Other sources of infection include direct contact from animals or the environment. Pregnant women can pass the infection to their babies during birth or through the placenta.