People across the U.S. were getting sick. Dozens were hospitalized. Two people died.

The cause was Listeria monocytogenes, a bacteria found in food. But no one knew the source until late last month, when Maryland health officials used a combination of old-fashioned detective work, modern biotechnology and a blender to nail the culprit in a Baltimore grocery store. Their findings triggered a nationwide recall of more than 7 million pounds of deli meat from a plant in Virginia.

Pinpointing the source of a foodborne illness is equivalent to “looking for a needle in a haystack,” said Sin Urban, division chief of the Division of Environmental Sciences Laboratories Administration. “So you can miss it at any stage.”

Maryland labs typically confirm about two dozen cases of listeria a year. But there may be far more cases of that pathogen and several other more common ones, such as salmonella and campylobacter. An estimated 48 million Americans get sick, 128,000 are hospitalized, and 3,000 die a year from foodborne illnesses, according to the U.S. Centers for Disease Control and Prevention.

A big problem with listeria is it can make people sick up to 70 days after consuming the contaminated food, said Shauna C. Henley, a University of Maryland food safety expert.

“Do you remember everything you ate or drank in last 24 hours, or after 30 days or 70 days?” asked Henley, an affiliate agent in the university’s Department of Nutrition and Food Science. “Figuring out what food item could have caused an illness can be super hard when people don’t remember. And older people, among the most vulnerable to foodborne illness, may have many other health issues that make their primary care physician accidentally overlook food altogether.”

When seven people got sick in Maryland, state epidemiologists interviewed them about what they’d eaten. Where did they shop?

The patients were generally elderly, with an average age of 74. Older adults, along with children and pregnant people, are at increased risk of having severe complications with listeria. But the patients’ age caught scientists’ attention for another reason, Urban said — they might have been eating something that younger generations wouldn’t.

It turned out that all of those afflicted had eaten liverwurst, a traditional German sausage made from pork or beef livers, in addition to two other meats, according to Urban. Staffers with the state’s Office of Food Protection, which regulates food establishments, went to a Baltimore grocery store to buy unopened packages of the products for scientists to examine. The meat had to be in sealed packaging to ensure they would find the source of the listeria, and not get a false positive from cross-contamination during processing, Urban said. (Listeria can spread on a meat slicer, for example.)

At the same time, state investigators knew the USDA was looking into a potential listeria outbreak at a Boar’s Head plant in Virginia. Officials didn’t say where they bought the samples, but according to the Boar’s Head website, its products are sold at Giant Foods, Graul’s Market, Eddie’s of Roland Park, Sprouts and Shoprites in the area.

Once they had the meats in tow, state officials sent them to Urban’s lab for testing.

Within a 3-pound loaf of liverwurst, the listeria wouldn’t have been uniformly distributed, Urban said. The deadly bacteria could have been hiding out in one end, or even in the middle. To ensure they were testing the whole sausage, lab scientists cut it into three pieces, placing each in a blender. Imagine, if you can, a liverwurst smoothie. They then tested multiple samples from the smoothie.

Bingo.

The liverwurst tested positive for listeria. Scientists immediately alerted the USDA, which then announced a recall for Boar’s Head liverwurst. But they didn’t stop there. Maryland scientists sent the isolated bacteria strands to another state laboratory for DNA sequencing.

Robert Myers calls the Maryland Department of Health Laboratories Administration, which he directs, “the CSI lab for infectious diseases.” It stores “fingerprints” for all kinds of organisms that make people sick. It’s part of a national network sponsored by the Centers for Disease Control called PulseNet.

Whenever someone gets sick with salmonella or another form of food poisoning, bacteria from their feces is taken to the state lab for testing and genetic sequencing. Findings from the state go into a worldwide database, which allows epidemiologists to find commonalities in outbreaks and where they are happening.

Myers’ scientists matched the exact listeria that Urban’s group identified in the liverwurst with the bacteria strains that had been found in sick patients in Maryland and beyond. In the past, they’ve used this technology to find vibrio bacteria in Venezuelan crab meat, E. coli in salads and salmonella in papayas.

“People don’t know a lot about it,” Myers said, “but it’s truly a lot of really highly skilled people working together to make all this come to fruition, to protect the food supply at the federal, state and local level.”

Henley said technology is evolving to make the lab work easier and more precise, but the Maryland staff should be commended for their swift identification of the pathogen so people could be warned to avoid the deli meat.

In the future, changes to laws and regulation, plus a push to update sanitation procedures and worker training, should also help prevent more people from getting sick. Food recalls are already increasing due to better upstream testing — not because there is a lot more contaminated food.

Still, she said, there are things people can do to help, such as keep refrigerators at a chilly 40 degrees, clean to prevent pathogen growth, consider prepackaged deli meats that never touch a potentially unsanitary deli counter slicer, and sign up for a grocery store loyalty card that issues you a warning when you’ve purchased a recalled item. Or, consumers can follow food recalls here.

That all will mean fewer needles the lab has to find in the haystack.