Mystery Parasite Outbreak Spreads Across the US as the CDC Hunts for a Source

Something is making Americans sick this summer, and health officials cannot yet say what. A microscopic parasite too small to see with the naked eye has left more than a hundred people battling a punishing intestinal illness across much of the country, from New York down to Texas and out to Alaska. Federal investigators have been chasing the source for weeks, running down clusters of cases in multiple states, and so far the trail keeps going cold.

What they do know points to food eaten right here in the US, since almost none of the sick had traveled anywhere. Beyond that, the picture grows murky fast, with a fast-moving Midwest cluster complicating an already tangled hunt. Here is what public health authorities have pieced together about a bug that turns an ordinary summer into a miserable one.

What The Numbers Show So Far

As of June 16, 2026, the CDC counted 145 cases of cyclosporiasis in people who caught the illness inside the US. Twenty of them landed in the hospital, though no one has died. Patients ranged in age from 5 to 86 years, with a median age of 42, and 61 percent were women. Doctors diagnose the illness only through a stool test, so many milder cases likely went unreported, meaning the real figure runs higher than what officials have logged.

A separate group of 45 people got sick after eating or drinking contaminated food or water while traveling outside the country, and three of them required hospitalization. Health authorities keep those travel-linked cases in a different column from the 145 domestic ones, since the two groups tell different stories about where the parasite came from. Combining them, the CDC tallied 190 cases total across both categories during the same May-to-June window.

Why The Source Is A Mystery

People usually pick up cyclosporiasis abroad, in tropical or subtropical regions, from food or water tainted with feces carrying the parasite. That pattern makes the current outbreak strange, because none of the 145 US patients reported any travel in the 14 days before they got sick. Something they ate at home made them ill, and officials cannot say what.

Person-to-person transmission does not happen with this parasite, which narrows the field considerably. Contaminated food eaten inside the country remains the leading explanation, yet the specific food has eluded investigators. The CDC put it plainly in its surveillance report, noting that “Local, state, and federal (CDC, FDA) public health authorities are investigating several clusters of cases in more than one state. Investigations to identify potential sources are ongoing.” Weeks into the effort, that statement still describes where things stand.

Not One Outbreak, But Many Clusters

A key distinction tends to get lost in coverage of this illness. The CDC has said there is no evidence of a single, multistate outbreak tying all the cases together. What the agency has instead is a surveillance count, a running tally of infections reported across the country. Some of those infections fall into clusters that the FDA is chasing through traceback investigations, while others connect to no common source at all.

Framing matters here, because “outbreak” suggests one contaminated product moving through the food supply and sickening people in a coordinated pattern. That is not what the data shows. Several separate investigations are running at once, each trying to work out whether a given group of cases shares a source. Some may turn out to be linked, and some may not.

Where The Cases Are Concentrated

New York has reported the most cases, somewhere between 31 and 80. Texas and Illinois follow, each with 11 to 30 cases. Other affected states include Alaska, Colorado, Connecticut, Florida, Georgia, Louisiana, Massachusetts, New Jersey, North Carolina, Ohio, Pennsylvania, Rhode Island, Tennessee, Virginia, and Wisconsin, along with Washington, DC. Cases cluster toward the East Coast, though they reach as far west as Texas, Colorado, and Alaska.

Health officials caution that the parasite has almost certainly spread beyond the states with confirmed cases. Because milder infections often clear up without a doctor visit, and because diagnosis requires a specific stool test that many sick people never get, the map of known cases undercounts the true reach. Places with zero reported cases may simply have zero tested cases.

A Fast-Growing Cluster In Michigan

Michigan changed the picture on July 1, after the CDC had already published its count of 145. State health officials there reported more than 170 cases across seven counties in just nine days, a sudden jump that arrived separately from the federal tally. Monroe County took the hardest hit at 70 cases, followed by Lenawee at 33 and Washtenaw at 21, with the rest spread across Wayne, Livingston, Shiawassee, and Jackson counties.

Dr. Natasha Bagdasarian, Michigan’s chief medical executive, warned that the count would likely climb. “Based on the unusual number of cases we have identified in a little over a week, we anticipate additional cases of illness being reported,” she said, adding that Michiganders with sudden, ongoing diarrhea should contact their health care provider and reach out to their local health department if other family members fell ill too. Officials had not yet confirmed whether every recent case belonged to the same outbreak, and an investigation into the cause was underway.

The Symptoms That Set It Apart

Symptoms usually appear about a week after infection, and the main one is watery, often explosive diarrhea. Loss of appetite, weight loss, cramping, bloating, increased gas, nausea, and fatigue tend to come with it. Less common effects include vomiting, body aches, headache, and a low-grade fever. Some people carry the parasite without feeling sick at all.

Physicians who treat the illness say timing separates it from more familiar stomach bugs. Dr. Irfan Hafiz, an infectious disease physician at Northwestern Medicine, compared it to a more sudden illness, telling NBC Chicago that with cyclosporiasis, “The symptoms are a bit less, but they go on longer. Noroviruses kind of hits you hard and then it’s done. This kind of just drags on for a longer period of time.” Without treatment, the CDC says symptoms can last anywhere from a few days to a month or more, and they may fade before returning. Fatigue can linger even after the stomach trouble stops.

How It Spreads And Which Foods Carry Risk

Cyclospora spreads when someone swallows food or water contaminated with infected feces. Fresh produce that resists cleaning tends to carry the highest risk, and past US cases have been tied to raspberries, cilantro, basil, mesclun and mixed greens, snow peas, and sweet peas. History offers cautionary examples. Basil imported from Mexico drove an outbreak in 2019, and tainted McDonald’s salads sickened nearly 400 people in 2018.

Dr. Sharon Welbel, chair of infectious diseases for Cook County Health, pointed to the produce that keeps turning up in these cases. Herbs like garlic and cilantro give the parasite a place to hide because their surfaces prove harder to clean, and delicate berries such as raspberries pose the same problem because washing them thoroughly without crushing them takes care. Welbel suggested soaking fragile items like basil or cilantro for a while before eating them. Hafiz added that some people may catch the parasite from swimming in lakes, ponds, or water parks. The bug does not pass from one person to another.

Treatment And What Officials Advise

Cyclosporiasis sometimes clears on its own, but it often calls for antibiotics, usually trimethoprim-sulfamethoxazole, sold under the brand names Bactrim, Septra, or Cotrim. A stool test remains the only way to confirm the diagnosis, so the CDC urges anyone with symptoms to see a health care provider for testing and treatment rather than waiting it out.

Prevention comes down to a few habits. Wash your hands regularly, wash produce before eating it, and cook food thoroughly. Anyone traveling to higher-risk regions, which include Central and South America, south and southeast Asia, the Middle East, and Africa, should stick to bottled water and sealed pasteurized products where possible. Michigan agriculture official Jennifer Bonsky reinforced the food-safety basics as the state worked its outbreak, pressing residents to wash their hands before and after preparing food and to wash produce before eating it.

Why This Season And What Comes Next

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Cases climb during warmer months, and the CDC treats May 1 through August 31 as cyclosporiasis season. In 2026, that window opened on May 1, right on schedule. Warmer temperatures have long coincided with a rise in infections, which is part of why officials watch this period so closely each year.

Scale gives some sense of what may lie ahead. More than 1,000 US cases turned up last year, so the current count will probably grow before the season ends in late August, with the Fourth of July gatherings arriving at an awkward moment. For now, the priority for the CDC and its state and federal partners is finding the source, or sources, and shutting them off before the numbers rise further. That work remains unfinished. Anyone dealing with sudden, lingering gastrointestinal illness should contact a health care provider, since testing and treatment can cut short an illness that otherwise tends to drag on for weeks.