Listeria monocytogenes is not the kind of bacteria that usually grabs headlines. It doesn’t spread from person to person. It doesn’t cause surges in hospitals. But it does something far more insidious: it lurks quietly in food production environments, contaminates products without obvious warning signs, and sickens people least equipped to fight it off. That includes older adults, people with weakened immune systems, and pregnant women. When it causes illness in those people, it can lead to death.
What Is Listeria and How Does It Cause Illness?
Listeria is a hardy bacterium. It can survive and even multiply in refrigerated environments where most other bacteria shut down or die. It thrives in moist, cool areas of food processing facilities and clings to surfaces in the form of a sticky biofilm, which is basically a slime layer that resists ordinary cleaning. Once it gets into a food product—whether it’s a sandwich, a shake, or deli meat—it can stay there, undetected, until someone eats it. For some people, that exposure is deadly.
When Listeria causes infection, it’s called listeriosis. In healthy adults, it may cause a mild gastrointestinal illness or nothing at all. But in people with weakened immune systems, the story is very different.
The bacteria can move from the gut into the bloodstream and even into the central nervous system, causing sepsis or meningitis. Pregnant women are also at high risk. They may experience only mild symptoms themselves but can suffer miscarriage, stillbirth, or severe infection of the newborn.
Patients require treatment with high-dose intravenous antibiotics, including ampicillin and gentamicin.
Outbreak 1: Listeria in Nutrition Shakes for Elderly Patients
The first outbreak began quietly in 2018. A few residents of long-term care facilities developed severe infections. It wasn’t immediately clear what tied them together. But at the end of 2024, after six new cases emerged with the same bacterial fingerprint, the CDC reopened the investigation. The common link turned out to be a frozen nutritional shake, one of those high-calorie supplements often prescribed to elderly or frail patients. Forty-two people were infected, 41 were hospitalized, and 14 died.
Outbreak 2: Listeria in Prepackaged Sandwiches Across the Western U.S.
The second outbreak, identified around the same time, hit a different population but followed a similar pattern. This time the contaminated product was pre-packaged sandwiches, distributed across California, Arizona, Nevada, and Washington by a company in San Fernando, California. These are the type of sandwiches I occasionally find myself buying when I’m on a long road trip or rushing through an airport to catch a flight. Ten people were hospitalized with severe listeriosis.
Why Listeria Is So Dangerous in the Food Supply
What connects these two outbreaks is the way the bacteria lingers on surfaces, in equipment, and in foods meant for people who are already vulnerable. And what makes listeriosis so tragic is how preventable it is. If we maintain rigorous food safety standards, keep up inspections, and fund the public health teams who detect the patterns that connect a death in one state to a sandwich in another.
In the next post, I’ll explain how we track outbreaks like these, and why that detective work is becoming harder to do.