antibiotics may be given to pregnant women even in non-invasive listeriosis. Mirena Nikolova, et al., states that applying antibiotics is crucial during the third trimester because cell-mediated immunity is reduced during this time. Pfaff and Tillet say that listeriosis can cause long-term consequences—including meningitis, preterm labor, newborn sepsis, stillbirths—when contracted during pregnancy. Oral therapies in less severe cases may include amoxicillin or erythromycin. Higher doses may be given to pregnant women to ensure penetration of the umbilical cord and placenta. Infected pregnant women may receive ultrasound scans to monitor the health of the fetus. Asymptomatic patients who have been exposed to Listeria typically are not treated, but are informed of the signs and symptoms of the disease and advised to return for treatment if any develop. Research Some Listeria species are opportunistic pathogens: L. monocytogenes is most prevalent in the elderly, pregnant mothers, and patients infected with HIV. With improved healthcare leading to a growing elderly population and extended life expectancies for HIV infected patients, physicians are more likely to encounter this otherwise-rare infection (only seven per 1,000,000 healthy people are infected with virulent Listeria each year). Better understanding the cell biology of Listeria infections, including relevant virulence factors, may lead to better treatments for listeriosis and other intracytoplasmic parasite infections. Researchers are now investigating the use of Listeria as a cancer vaccine, taking advantage of its "ability to induce potent innate and adaptive immunity." See also 2008 Canada listeriosis outbreak 2011 United States listeriosis outbreak 2017–2018 South African listeriosis outbreak 2018 Australian rockmelon listeriosis outbreak List of foodborne illness outbreaks References Further reading Food and Drug Administration (FDA). 2005. "Foodborne Pathogenic Microorganisms and Natural Toxins Handbook: The ìBad Bug Book" Food and Drug Administration, College Park, MD. Accessed: 1 March 2006. Frank, J. F. 2001. "Microbial attachment to food and food contact surfaces". In: Advances in Food and Nutrition Research, Vol. 43. ed. Taylor, S. L. San Diego, CA. Academic Press., Inc. 320–370. Nikolova, M., Todorova, T. T., Tsankova, G., & Ermenlieva, N. (2016). А possible case of а newborn premature baby with Listeria monocytogenes infection. Scripta Scientifica Medica, 48(2). Pfaff, N. F., & Tillett, J. (2016). Listeriosis and Toxoplasmosis in Pregnancy: Essentials for Healthcare Providers. The Journal of perinatal & neonatal nursing, 30(2), 131. Ross, H. (2015). Food Hygiene: Rare Burgers. Eur. Food & Feed L. Rev., 382. USDA-FSIS. (United States Department of Agriculture – Food Safety and