A surge in cyclospora infections across the United States has brought renewed attention to the parasite’s behavior and the challenges it poses for patients and health professionals. Cyclospora, an intestinal parasite transmitted through food or water contaminated with human feces, has historically caused relatively few outbreaks in the U.S. However, this year has seen a significant increase in cases, bringing concerns about recurring symptoms and treatment effectiveness.

Since May 1, 2026, federal health authorities have confirmed approximately 10,400 cyclospora infections, with more than 12,000 additional suspected cases under investigation nationwide. This marks a considerable rise compared to the annual average of around 2,800 cases reported since 2016.

Cyclospora infections often cause severe diarrhea, stomach cramps, nausea, and fatigue, and the symptoms can fluctuate over weeks. Medical experts say patients frequently experience periods of improvement followed by sudden relapses, which can be distressing. “They think they’re getting better and then, boom, it comes back again,” explained Dr. Roy Gulick, chief of infectious diseases at Weill Cornell Medicine. While some individuals recover without treatment, others suffer intermittent or continuous diarrhea lasting more than a month.

Treatment typically involves the antibiotic trimethoprim-sulfamethoxazole (Bactrim), which can reduce the duration and severity of symptoms. Still, there is no clear medical consensus on what constitutes a relapse versus the natural ebb and flow of the illness. Research is limited, but a small 2011 study in Quebec observed that 25% of infected individuals experienced one or more symptom recurrences within three months, regardless of antibiotic use.

Recurrence may occur for several reasons. In some cases, the initial infection may not clear entirely, especially in older adults or those with weakened immune systems, according to Dr. Anurag Malani, an epidemiologist at Trinity Health Ann Arbor Hospital. Interrupted antibiotic courses pose another risk, as patients who stop medication prematurely may leave residual parasites behind, noted Dr. Alexander Chow, chief of infectious disease at NYC Health & Hospitals Woodhull.

Additionally, reinfection is plausible, since the human body does not develop strong immunity toward cyclospora. The parasite targets intestinal lining cells rather than deeper body tissues, limiting antibody response. “It is entirely possible to have a case of cyclospora, get over it, get better, get reinfected and get another case of cyclospora,” said Dr. Linda Yancey, an infectious disease specialist at Memorial Hermann Health System in Houston.

Some symptoms that appear to represent a relapse might actually stem from post-infectious complications such as irritable bowel syndrome (IBS), where residual gut damage causes ongoing digestive issues even after the parasite is cleared. Such cases typically improve over time, according to Dr. Cory Fisher, a family medicine physician with the Cleveland Clinic in Ohio.

Patients affected this summer report significant disruptions to daily life. Angie Onofre, a 25-year-old I.T. intern in Brooklyn, experienced a severe return of diarrhea weeks after initially recovering and required hospitalization for dehydration. Brittani Leone, a nurse from Pittsburgh, endured more than two weeks of fluctuating symptoms before starting antibiotics, after which she began to improve gradually but still felt weak and foggy-headed. Jill Woolley, 55, from Kansas City, Missouri, initially mistook her illness for a virus until her symptoms returned during a vacation, prompting her to seek treatment and subsequently recover.

With the outbreak ongoing, health officials continue to stress the importance of completing prescribed antibiotic courses and taking precautions to avoid exposure to contaminated food or water. Further research is needed to fully understand cyclospora’s clinical course and optimal management strategies amid the historic spike in cases this year.