According to the US Department of Agriculture (USDA), more than 1.8 million bacteria units are lurking in the average home refrigerator. Human eyes cannot see these health dangers lurking in your refrigerator. Some bacteria degrade your food prematurely, and others carry illnesses like Salmonella and Listeria.

These invisible, pathogenic microbes can cause respiratory and urinary infections, food poisoning, and even miscarriages among pregnant women. The highest concentrations of bacteria can be found in the vegetable drawer, the meat drawer, and on shelves where dairy products are stored.

Most bacteria thriving in refrigerators develop from unwashed vegetables or cross-contamination, so experts recommend covering all food before placing it inside. The USDA states consumers should put leftovers in airtight containers to avoid bacterial growth. Place these containers on one of the upper shelves for added protection, and do not store eggs on the door but in the carton on a shelf.

Spoilage bacteria can grow at low temperatures, such as in the refrigerator, and are detected when the food spoils.

Pathogenic bacteria flourish in the ‘Danger Zone,’ between 40 and 140 degrees Fahrenheit. This group causes food-borne illnesses but typically does not impact food’s taste, smell, or appearance. Listeria is a pathogenic bacteria that primarily affects pregnant women, children, and the elderly. In pregnant women, listeriosis can cause miscarriage, stillbirth, and severe illness or death of newborn babies. Listeria can survive on cold surfaces and multiply slowly at 24 degrees Fahrenheit. Listeria can be found throughout the fridge, specifically where meats and dairy items are stored.

Symptoms of food-borne infections include fever, stiff neck, confusion, weakness, and vomiting, sometimes preceded by diarrhea. The incubation period can be 3 to 70 days, and the illness may last days or weeks, depending on the person’s health before the disease settles in.