Although most cases occur sporadically, foodborne outbreaks do occur. Listeria bacteria are widely distributed in nature and can be found in water and soil. Infected animals may also serve as sources. Unlike other organisms, listeria can be spread by several different methods. In newborn infections, the organism may be transmitted from mother to fetus in utero, or directly to the fetus at the time of birth.
Infection is also possible by inhaling the organism.
Because listeriosis can affect many different parts of the body, the symptoms vary. For meningoencephalitis, the onset can be sudden with fever, intense headache, nausea, vomiting and signs of meningeal irritation. In other body locations, various types of lesions at the site of infection are the primary symptom. In most cases, listeria infection causes fever and influenza-like symptoms resembling many other illnesses. Listeriosis has an extremely variable incubation period.