The latest reason to side-eye the salad aisle is microscopic.

Cases of an intestinal illness that can leave people sick for weeks are rising across the United States, and health officials still do not know exactly what people are eating or drinking that is causing the infections. So far, the investigation has produced thousands of cases, several possible leads, and one deeply unhelpful answer for anyone standing in the produce section: The culprit is still unclear.

Since May 1, the Centers for Disease Control and Prevention has received reports of 1,645 confirmed domestically acquired cases of cyclosporiasis across 34 states. The agency is also reviewing more than 5,100 additional cases that may be connected. At least 141 people have been hospitalized, and no deaths have been reported.

The official count may represent only a fraction of the illnesses. The confirmed total is already significantly higher than it was at the same point last year, when 249 cases had been reported nationally. People with milder symptoms may never seek medical care, cyclosporiasis requires a specific laboratory test, and several weeks can pass before illnesses appear in federal data.

Michigan has emerged as the center of the surge. The state reported 3,762 cases as of July 15, a remarkable increase for a state that typically records only 40 to 50 cases during an entire year.

Health officials believe more than 400 infections across Michigan, Ohio, Kentucky, and West Virginia may be part of one connected multistate outbreak. Other clusters are being investigated separately, meaning several contaminated foods or sources may be circulating all at the same time.

The outbreak appears to be spreading through produce, while uncertainty—and memes—spread just as quickly across the internet.

What is cyclosporiasis?

Cyclosporiasis is an intestinal illness caused by Cyclospora cayetanensis, a microscopic parasite. People generally become infected after consuming food or water contaminated with the parasite, which has frequently been associated with raw fruits, vegetables, and herbs.

The parasite is tiny, but the illness it causes can be anything but brief. Its hallmark symptom is prolonged watery diarrhea, but people may also experience stomach cramps, bloating, nausea, fatigue, gas, loss of appetite, and weight loss. Some people develop body aches, vomiting, or a low fever, according to the CDC.

Symptoms usually begin about one week after exposure but can appear anywhere from two days to two weeks later, making it particularly easy to mistake for a stomach bug that has finally packed its bags. Without treatment, they can continue for several weeks or more than a month, making cyclosporiasis considerably more persistent than the average short-lived stomach bug.

The parasite does not usually pass directly from one person to another. After leaving an infected person’s body, Cyclospora must remain in the environment before it becomes infectious, so health officials are focusing on contaminated food and water rather than casual contact with sick people.

Lettuce and other produce are under scrutiny

Investigators have not identified a specific food, farm, processor, supplier, restaurant, or grocery store responsible for the increase. In Michigan, however, early interviews with people who became sick have repeatedly pointed investigators toward lettuce and salad greens.

The Michigan Department of Health and Human Services has completed more than 1,000 interviews with people who tested positive. Lettuce regularly appeared in their recent food histories, although officials have stressed that other foods remain possible and that no particular variety, grower, or supplier has been identified.

Tracing the source is complicated by the parasite’s incubation period. Someone may not develop symptoms until two weeks after eating a contaminated product and may not get tested until even later. By the time an investigator asks what that person ate, they may be trying to recall an unremarkable salad or sandwich from nearly a month earlier.

Produce distribution adds another hurdle. The same lettuce may be sent to restaurants, grocery stores, cafeterias, and other businesses across several states, exposing people with no obvious connection to the same product.

The Food and Drug Administration currently lists several active Cyclospora investigations involving products not yet identified. The agency has begun tracing products through their supply chains, but it has not announced a recall or issued a nationwide warning against a specific item connected to the broader spike.

Previous U.S. outbreaks have been tied to raspberries, basil, cilantro, snow peas, lettuce, bagged salad mixes, and other fresh produce. In 2018, McDonald’s pulled salads from thousands of restaurants during an outbreak eventually traced to a salad supplier.

State, federal health officials probe possible Taco Bell link

Taco Bell entered the story before health officials publicly named any restaurant. Last week, customers began sharing photos of notices posted at locations around Metro Detroit saying the restaurants were unable to serve lettuce, cilantro-onion blends, pico de gallo, and guacamole because of a “nationwide recall.” The signs quickly circulated on Reddit and through local news reports, even though the FDA had not announced a nationwide recall involving those ingredients.

That discrepancy immediately raised questions. The company did not initially identify which locations were affected, where the ingredients had come from, or why the action appeared concentrated in Michigan, the state at the center of the outbreak.

The local speculation became a national story on July 14, when The Washington Post reported that federal and state health officials were investigating whether Taco Bell restaurants played a role in at least part of the outbreak. The report cited two people familiar with the ongoing investigation, one of whom said some patients had told investigators they ate at Taco Bell before becoming sick. Others had not, indicating that the chain could not account for the broader surge even if investigators eventually establish a connection.

The Taco Bell lead also lined up with what Michigan investigators were finding elsewhere. After interviewing more than 1,000 people who tested positive, state officials said lettuce or salad greens repeatedly appeared in their food histories. That does not identify Taco Bell’s lettuce, or any specific lettuce, as the source, but it does explain why they “voluntarily and temporarily removed limited ingredients at select restaurants” as a precaution.

Still, the combination of a recognizable fast-food chain, mysterious “recall” signs, and a reported federal investigation was enough to send Crunch Wrap Supreme-fans into a frenzy. Shares of parent company Yum Brands fell as much as 4.5 percent after the Post report.

Can washing produce remove Cyclospora?

Health officials are not advising everyone in the country to stop eating fresh produce. Instead, they recommend extra care, particularly for lettuce and salad greens, in areas with high case numbers.

The CDC recommends washing fruits and vegetables under clean running water before eating or preparing them, including produce labeled prewashed. Firm items such as cucumbers, melons, and potatoes can be rubbed by hand or scrubbed with a clean produce brush.

Washing may reduce contamination, but it cannot guarantee that Cyclospora has been removed, particularly from leafy greens, herbs, and delicate berries. Soap, bleach, detergent, and household cleaning products should not be used on food, and the parasite can resist some chemical sanitizers.

Michigan officials recommend purchasing whole heads of lettuce rather than precut lettuce, bagged mixes, or salad kits. Consumers can remove the outer two or three layers and wash the remaining inner leaves under running water.

Cooking offers more protection. Heating food to at least 158 degrees Fahrenheit (70 degrees Celsius) kills Cyclospora, making cooked vegetables a lower-risk choice than raw produce. Peeling fruits and vegetables after washing them can also help remove potentially contaminated outer surfaces.

What to do if you develop symptoms

People who experience persistent or recurring watery diarrhea should contact a healthcare provider, especially if they are also dealing with dehydration, severe fatigue, nausea, appetite loss, or weight loss.

Patients may need to mention cyclosporiasis specifically. Standard tests for diarrhea do not always include Cyclospora, and the parasite can be difficult to detect in a single stool sample. The CDC has asked clinicians to request Cyclospora testing when they suspect an infection, particularly for patients with prolonged or relapsing symptoms during the spring and summer.

The recommended treatment is a seven- to 10-day course of trimethoprim-sulfamethoxazole, an antibiotic combination commonly sold under the brand names Bactrim and Septra. Staying hydrated is also important because frequent diarrhea can lead to dehydration.

Officials expect case numbers to continue rising while investigators work backward through weeks of meals and complex supply chains for produce. Until they identify the source, washing a salad is still worthwhile, but cooking dinner may offer a little more peace of mind.

Where to find the latest case numbers

The CDC’s cyclosporiasis surveillance page provides the latest national case count and the number of states reporting infections, while its outbreak page tracks the cluster currently linked to cases in Michigan, Ohio, Kentucky, and West Virginia. Because federal numbers can lag behind state reporting, readers should also check their state or local health department’s website for more recent regional totals.

The FDA’s active outbreak-investigation table is the best place to check whether officials have identified a food, launched a traceback investigation, or announced a recall.