The Centers for Disease Control and Prevention has linked an outbreak of cyclosporiasis — an illness caused by a parasite transmitted through food or water contaminated with feces — to shredded iceberg lettuce served at Taco Bell locations in Indiana, Kentucky, Michigan, Ohio and West Virginia.

The C.D.C. is currently advising consumers to not eat shredded lettuce served at Taco Bell restaurants in those states.

According to two federal officials who declined to be named, the C.D.C. has found that Taylor Farms, one of the largest producers of fresh lettuce and vegetables in the country, supplied the lettuce to Taco Bell. Its bagged lettuce and salad kits are sold in many grocery stores in the United States, including Safeway, Whole Foods, Target and Walmart.

In a statement released Friday afternoon, Taylor Farms said that no Taylor Farms-branded salads or kits were associated with the outbreak, and that none of their branded salad kits contained iceberg lettuce. They said they were voluntarily removing all of the iceberg lettuce that was sourced from central Mexico.

On Thursday, the Food and Drug Administration said that other “brands, restaurants, retailers or distribution channels” might still be named as the investigation into other cyclosporiasis outbreaks and illnesses across the country continued.

Dr. Natasha Bagdasarian, who leads the health department in Michigan, said that no matter where people are eating, whether at home or in restaurants, they should be mindful of where the lettuce they’re eating was sourced from — whether it came from a bag or was prepared from a whole head — because the investigation had not been concluded.

She recommended avoiding bagged lettuce and salad mixes, and prioritizing whole-head lettuce that was washed after the outer leaves and damaged areas were removed.

Dr. Bagdasarian was reluctant to identify a specific type of lettuce or food setting as a potential source, warning that the investigation was not yet complete. Cross-contamination can occur when different types of produce greens are washed and mixed together, she said.

“We are seeing exposure potentially across diverse settings,” she said, adding, “I think it could be premature to say that we believe this is just one product.”

Dr. Teena Chopra, a professor of medicine at Wayne State University School of Medicine in Detroit, said the outbreak underscored how interconnected the food supply is. “A single contaminated product can reach multiple states before anyone gets sick, especially since cyclospora has such a long incubation period,” she said. It can take from a couple of days to weeks after an infection for someone to develop symptoms, which include severe, watery diarrhea.

“At this point, we’re still kind of in a waiting game,” as the investigation plays out, said Keith R. Schneider, a professor of food safety at the University of Florida.

People with weakened immune systems are at greater risk for infection with cyclospora and other food-borne illnesses, so they may consider avoiding all raw vegetables, fruits and herbs, especially for the time being. A safer bet is to cook produce thoroughly or consume frozen or canned versions, said Ellen Shumaker, director of outreach for the Safe Plates Program at North Carolina State University.

Barbara Kowalcyk, director of the Institute for Food Safety and Nutrition Security at George Washington University, said that “it’s probably overkill to avoid all leafy greens” — which include kale, spinach, cabbage and collard greens.

Dr. Chopra said that she personally was comfortable eating lettuce that wasn’t iceberg lettuce, but she added that she would avoid eating salads in restaurants until all of the affected produce had been recalled or removed from restaurant and grocery store shelves.

The United States has had cyclospora outbreaks in recent years, though with fewer cases than the current spread. It’s likely that some cases reported across the country are not linked to the lettuce served at Taco Bell, Dr. Schneider said.

People who are in high-risk groups for food-borne illnesses may want to take extra precautions like cooking raw vegetables — not only because of cyclospora, but also because of a variety of pathogens that regularly spread in the United States, Dr. Kowalcyk said. Those most vulnerable include children, older adults and those who are pregnant or immunocompromised.