Listeria species are Gram-positive bacilli and are typified by L.monocytogenes, the causative agent of listeriosis, which is a relatively rare bacterial infection and occurs primarily in newborn infants, elderly patients, and patients who are immunocompromised. The symptoms of listeriosis usually last 7-10 days. The most common symptoms are fever and muscle aches. Nausea and diarrhea are less common symptoms. If the infection spreads to the nervous system it can cause meningitis.