Incidence in 2004–2005 was 2.5–3 cases per million population a year in the United States, where pregnant women accounted for 30% of all cases.^[10] Of all nonperinatal infections, 70% occur in immunocompromised patients. Incidence in the U.S. has been falling since the 1990s, in contrast to Europe where changes in eating habits have led to an increase during the same time. In Sweden, it has stabilized at around 5 cases per annum per million population, with pregnant women typically accounting for 1–2 of some 40 total yearly cases.^[11]
There are four distinct clinical syndromes:

• Infection in pregnancy: Listeria can proliferate asymptomatically in the vagina and uterus. If the mother becomes symptomatic, it is usually in the third trimester. Symptoms include fever, myalgias, arthralgias and headache. Miscarriage, stillbirth and preterm labor are complications of this infection. Symptoms last 7–10 days.
• Neonatal infection (granulomatosis infantisepticum): There are two forms. One, an early-onset sepsis, with Listeria acquired in utero, results in premature birth. Listeria can be isolated in the placenta, blood, meconium, nose, ears, and throat. Another, late-onset meningitis is acquired through vaginal transmission, although it also has been reported with caesarean deliveries.
• Central nervous system (CNS) infection: Listeria has a predilection for the brain parenchyma, especially the brain stem, and the meninges. It can cause cranial nerve palsies, encephalitis, meningitis, meningoencephalitis and abscesses. Mental status changes are common. Seizures occur in at least 25% of patients.
• Gastroenteritis: L. monocytogenes can produce food-borne diarrheal disease, which typically is noninvasive. The median incubation period is 21 days, with diarrhea lasting anywhere from 1–3 days. Patients present with fever, muscle aches, gastrointestinal nausea or diarrhea, headache, stiff neck, confusion, loss of balance, or convulsions.

Particular strains of Listeria monocytogenes are able to invade the heart, leading to serious and difficult-to-treat heart infections. About 10 percent of serious listeria infections involve cardiac infections that are difficult to treat, with more than one-third proving fatal. A strain of listeria had been isolated from a patient with endocarditis (infection of the heart). Usually with endocarditis, there is bacterial growth on heart valves, but in this case the infection had invaded the cardiac muscle. When mice were infected with either the cardiac isolate or a lab strain, 10 times as much bacteria were found in the hearts of mice infected with the cardiac strain. In the spleen and liver, organs that are commonly targeted by listeria, the levels of bacteria were equal in both groups of mice. While the lab-strain-infected group often had no heart infection at all, 90 percent of the mice infected with the cardiac strain had heart infections. Only one other strain of listeria out of 10 acquired seemed to also target the heart. The results suggest that these two cardiac-associated strains display modified proteins on their surface that enable the bacteria to more easily enter cardiac cells, targeting the heart and leading to bacterial infection