ListeriaareGram-positivebacilliandarewidespreadintheenvironment.Itcanbefoundinsoil,dust,mud,vegetation,silage,sewageandmostanimals.Ithasbeenfoundin5%ormoreofnormalhealthypeople,usuallyinthegut.Ithastheunusualcharacteristicofbeingabletogrow,albeitslowly,attemperaturesaslowas0°C.ThereareseveraldifferentsubtypesofL.monocytogenes(themaincauseofdisease)andL.innocua,whichisverysimilarbutapparentlydonotcausediseaseinman.