Listeria monocytogenes is a bacterium that can cause foodborne illness in a person who eats a food item contaminated with it. Symptoms of infection may include fever, muscle aches, gastrointestinal symptoms such as nausea or diarrhea. If it spreads to the nervous system symptoms may include headache, stiff neck or confusion. The illness primarily impacts pregnant women and adults with weakened immune systems. Most healthy adults and children rarely become seriously ill.