The Incidence of Listeria Infections
How common are Listeria infections?
Listeria bacteria are found widely in the environment in soil, including in decaying vegetation and water, and may be part of the fecal flora of a large number of mammals, including healthy human adults. [4, 18] According to the FDA, “studies suggest that 1-10% of humans may be intestinal carriers of Listeria.”  Another authority notes that the “organism has been isolated from the stool of approximately 5% of healthy adults.”  Overall, seasonal trends show a notable peak in total Listeria cases and related-deaths from July through October. 
Ingested by mouth, Listeria is among the most virulent foodborne pathogens, with up to 20% of clinical infections resulting in death.  These bacteria primarily cause severe illness and death in persons with immature or compromised immune systems. [13, 18] Consequently, most healthy adults can be exposed to Listeria with little to no risk of infection and illness. [4, 11]
A study published in 1995 projected Listeria infection-rates to the U.S. population, suggesting that an estimated 1,965 cases and 481 deaths occurred in 1989 compared with an estimated 1,092 cases and 248 deaths in 1993, a 44% and 48% reduction in illness and death, respectively.  In comparison, a USDA study published in 1996 estimated that there had been 1,795-1860 Listeria-related cases in 1993, and 445-510 deaths, with 85-95% of these attributable to the consumption of contaminated food.  Listeriosis-related mortality rates decreased annually by 10.7% from 1990 through 1996, and by 4.3% from 1996 through 2005. 
Among adults 50 years of age and older, infection rates were estimated to have declined from 16.2 per 1 million in 1989 to 10.2 per 1 million in 1993.  Perinatal disease decreased from 17.4 cases per 100,000 births in 1989 to 8.6 cases per 100,000 births in 1993.  Neonatal infections are often severe, with a mortality rate of 25-50%. 
According to the CDC’s National Center for Zoonotic, Vector-Borne, and Enteric Diseases:
Listeriosis was added to the list of nationally notifiable diseases in 2001. To improve surveillance, the Council of State and Territorial Epidemiologists has recommended that all L. monocytogenes isolates be forwarded to state public health laboratories for subtyping through the National Molecular Subtyping Network for Foodborne Disease Surveillance (PulseNet). All states have regulations requiring health care providers to report cases of listeriosis and public health officials try to interview all persons with listeriosis promptly using a standard questionnaire about high risk foods. In addition, FoodNet conducts active laboratory- and population-based surveillance. 
In 2006, public health officials from 48 states reported 1,270 foodborne disease outbreaks, with a confirmed or suspect source in 884 of the outbreaks (70%).  Only one of the outbreaks with a confirmed source was attributed to Listeria, with this outbreak involving eleven hospitalizations and one death.  The next year, of 17,883 lab-confirmed infections, the CDC attributed 122 to Listeria.  In 2009, there were 158 confirmed Listeria infections, representing an incidence-rate of .34 cases for every 100,000 persons in the United States.  Such data revealed an incidence-rate of 0.27 cases per 100,000 persons, a decrease of 42% compared with 1996—1998.  But, according to CDC’s Technical Information website, it is estimated that there are 1,600 cases of Listeria infection annually in the United States, based on data through 2008. 
The 2009 numbers represented a reported 30% decrease in the number of infections compared to the 1996—1998 rates of infection.  Although the nature and degree of underreporting is subject to dispute, all agree that the confirmed cases represent just the tip of the iceberg. [6, 13] Indeed, one study estimates the annual incidence rate for Listeria to be around 1,795-1,860 cases per 100,000 persons, with 445-510 of the cases ending in death. 
Finally, in a study of FoodNet laboratory-confirmed invasive cases—where infection is detected in blood, cerebrospinal fluid, amniotic fluid, placenta or products of conception—the number of listeriosis cases decreased by 24% from 1996 through 2003.  During this same period, pregnancy-associated disease decreased by 37%, while cases among those fifty years old and older decreased by 23%.