It is transmitted by animal contact, unpasteurized milk, or cheese made from unpasteurized milk.
Raw vegetables can also transmit the disease.
The organism initially colonizes the intestinal epithelium, then enters the macrophages and spreads hematogenously. It enters cells by induced phagocytosis, then escapes the phagosome by means of the virulence factor, listeriolysin, and grows in the cytoplasm. Using the cell's actin transport system, it spreads directly from cell to cell. Newborns can acquire the infection in utero or during passage through the birth canal, and are acutely ill with multiple organ involvement and symptoms of either pneumonia or meningitis. Infants between 1 and 4 weeks of age are also highly susceptible to infection by Listeria, but in this case the organism is more likely spread by person-person contact and manifests itself as meningitis or sepsis.