An infectious disease of humans and animals caused by the bacteria Listeria monocytogenes and L. ivanovii. Both humans and animals can be carriers, which excrete the bacterium in feces. Sheep, goats, and cattle can excrete the bacteria in milk, without clinical symptoms of mastitis. The most important pathway of infection is probably through food. Listeria monocytogenes has frequently been isolated from grass silage (fermented fodder), especially from silage of inferior quality, which is an important source of infection in ruminants. Relatively few animals develop clinical disease, but a high proportion can be latent carriers. Farm leys often are fertilized with manure, and the bacteria in infected manure may be distributed on plants and in soil. During forage harvesting, soil and dung may be brought into the silo together with the grass, thus introducing the bacteria into the silage. Listeria monocytogenes is able to multiply in silage with a high pH, which probably is the main source of infection in ruminants.