Store-bought ground beef often contains a variety of bacteria that can make humans sick and is resistant to the drugs used to treat it, according to new data from Consumer Reports.

While most bacteria in meat can be killed when cooked correctly, many Americans prefer to eat their meat rare, putting them at a greater risk for illness—especially when the meat comes from conventionally raised cows, which are treated with antibiotics and hormones, according to a new Consumer Reports study. The study found that nearly 20% of ground beef in the U.S. tested from conventionally raised cows had bacteria resistant to three or more classes of antibiotics. Only 9% of ground beef that was sustainably made had antibiotic-resistant bacteria.

For the report, Consumer Reports purchased and tested 300 packages of conventionally and sustainably produced ground beef sold in stores around the U.S. The meat was tested for five common types of bacteria that can be found in beef: Clostridium perfringens, E. coli, Enterococcus, Salmonella and Staphylococcus aureus. Bacteria of some kind was found in all of the beef samples, though sustainably produced beef was less likely to have harmful strains.

More than 80% of conventional ground beef had two types of bacteria and nearly 20% of the samples contained C. perfringens, which causes close to a million cases of food poisoning every year. “There’s no way to tell by looking at a package of meat or smelling it whether it has harmful bacteria or not,” Urvashi Rangan, executive director of the Center for Food Safety and Sustainability at Consumer Reports, said.“You have to be on guard every time.”

The research also found that 10% of the samples had a strain of S. aureus that produces a toxin that can make people ill and is not killed even when the meat is cooked properly. Still, cooking meat at 160 degrees Fahrenheit should kill most bacteria.

The findings suggest that consumers may want to look for ground beef that’s sustainably produced, with labels reading “no antibiotics,” “grass-fed,” and “organic,” according to Consumer Reports. Consumer Reports says “grass-fed organic” may be one of the best labels to go by since it means the cattle eat organic grass and forage and do not receive antibiotics or hormones.

United States Department of Agriculture (USDA) secretary Tom Vilsack said in a statement that the agency has put tight food safety standards in place over the last six years to avoid public health problems. “Measures taken to improve ground beef safety include a zero-tolerance policy for six dangerous strains of E. coli, better procedures for detecting the source of outbreaks, improved laboratory testing, and more. USDA’s food safety inspectors work in every meat facility, every day, to reduce illnesses across all products we regulate, and we’re proud to report that illnesses attributed to those items dropped by 10% from 2013 to 2014,” he said.

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