Sometimes bacteria are our friends, helping us digest dairy and ensuring immunity toward harmful viruses and illnesses. In the case of Listeria, however, it's a totally differet story.

Listeria, a food-borne bacteria, has been found to infect 90 percent of individuals with weakened immunity in the United States. This includes pregnant women, their newborns, and adults over the age of 65. The Centers for Disease Control and Prevention (CDC) has stated that Listeria infections are the third leading cause of death by food poisoning while food-borne illnesses cause about 3,000 deaths in the United States.

Food poisoning often occurs when one eats food contaminated with bacteria. Often, cooking will spare us from exposure to these bacteria. For example, Salmonella, a bacteria often found in eggs, dairy, meat, and poultry, can be easily killed by proper cooking or storage of the food products most contaminated. Often, eggs and meats are safe to eat after they have been cooked.

However, Listeria works in far more nefarious ways and often cannot be killed by cooking.

Listeria also tends to contaminate foods one would not think to cook. Milk, deli meats and hot dogs, cheeses, smoked seafood, fruits, and vegetables are among these. Because of this, Listeria outbreaks are hard to control, as infections can come from a number of places and symptoms are often delayed.

Listeria can also infect equipment and appliances, like deli meat slicers, carving boards in factories or grocery stores, and blenders and knives at home. Most horrifyingly, it can even proliferate on infected foods even if they are in the refrigerator. Use of such appliances and foods can be lethal, as one in six people get food-borne illness each year and one in five of these people will die.

Between 2009 and 2011, there were 1,651 incidences of Listeria infection by food poisoning, as reported by the CDC's Morbidity and Mortality Weekly Report. Of these, 58 percent of cases were among the elderly, and 292 cases affecting pregnant women resulted in miscarriages of their fetus. Not surprisingly, cheese was implicated in half of the cases that occurred in that two-year period. Similarly, contaminated cantaloupes were found to cause one of the deadliest food-borne outbreaks in the United States; 147 people were infected and 33 people died.

A major issue with Listeria infection is that it further weakens one's immune system. People with already weakened immune systems, such as those with cancer, diabetes, kidney failure, hepatitis, and HIV/AIDS, or the elderly as well as expecting mothers, are made more vulnerable to meningitis, miscarriage, and bacterial infection of the blood.

The annual incidence of Listeria infection from 2009 to 2011 has indicated that there has been little progress toward reducing the occurrence. As a result, the Food and Drug Administration (FDA), in 2011, carefully crafted the Food Safety Modernization Act (FSMA). The FSMA gives the FDA the authority to regulate food facilities, recall foods they have found to be infected by Listeria bacteria, establish higher standards for keeping produce safe, and oversee imported foods. In our own homes, we can carefully choose foods we purchase; avoiding unpasteurized or soft cheeses, refrigerating leftovers and only using them for three to four days, heating deli meats and hot dogs before consumption, and ensuring the refrigerator is set to 40°F and the freezer to 0°F to prevent bacterial growth.

Source: U.S. Centers for Disease Control and Prevention. Vital Signs: Listeria Illnesses, Deaths, and Outbreaks - United States, 2009-2011. Morbidity and Mortality Weekly Report. 2013.