Two weeks after Angie Onofre thought she’d finally recovered from the worst intestinal illness of her life, she was snacking on popcorn when she felt a familiar urge to run to the bathroom.
“Immediately I’m like, ‘Oh no,’” said Ms. Onofre, 25, an I.T. intern in Brooklyn. The severe diarrhea she thought had been cured with the antibiotic Bactrim was back with a vengeance. Her symptoms got so bad that she went to the hospital, where a second cyclospora test confirmed that the parasite was still in her stool and she received I.V. fluids to rehydrate her.
In the midst of the worst cyclospora outbreak in U.S. history, health experts are grappling with how the parasite — which is spread through food or water contaminated with human feces — behaves in the gut. Before the latest surge in infections, it’s been rare to have cyclospora outbreaks at anywhere near this scale in the United States. As a result, there has been little research on how a typical infection behaves.
What is clear, health experts say, is that the symptoms can wax and wane, sometimes repeatedly over weeks. That “can be quite disturbing for patients,” said Dr. Roy Gulick, chief of infectious diseases at Weill Cornell Medicine. “They think they’re getting better and then, boom, it comes back again.”
Brittani Leone, 31, a nurse in Pittsburgh, said a cyclospora infection hit her “out of nowhere” on July 13. She had just finished dinner with her husband when she lurched toward the bathroom. For five days, she said, she stayed more or less anchored to the toilet, able to eat only Popsicles and toast. When the symptoms faded, she celebrated by ordering Italian wedding soup. Less than an hour after eating, she said, “it all started again.” She endured another week of symptoms that came and went, including a resurgence of diarrhea.
About two weeks after her initial symptoms, she got an antibiotic and quickly started to feel better. She’s still weak, she said, as if her limbs are too heavy, and her head feels foggy. She took a leave of absence from work on her doctor’s recommendation. But she has been free of diarrhea since she started treatment.
Since 2016, the Centers for Disease Control and Prevention has reported an average of about 2,800 cyclospora cases per year in the United States; in 2026, the agency confirmed around 10,400 cases since May 1 and is investigating more than 12,000 additional cases in states across the country.
Dr. Cory Fisher, a family medicine physician with the Cleveland Clinic in Ohio, said he didn’t have a good sense of how often cyclospora symptoms recur. During a typical year, he said, he sees cyclospora patients “almost never.” This summer, he’s treated about two dozen of them.
Cyclospora infections can vary in duration and severity, Dr. Gulick said. Some people fight off the parasite without treatment and with minimal symptoms. Other people end up with diarrhea — either constant or intermittent — for more than a month. On average, symptoms last about three weeks without treatment, Dr. Gulick said, although antibiotics can shorten the duration.
There isn’t a formal definition for what counts as a relapse, or clarity on how to differentiate a relapse from the typical ebb and flow of symptoms.
When people do have relapsing symptoms, it’s not always clear why, Dr. Gulick said. In one small study published in 2011, researchers in Quebec found that among 100 people who caught cyclospora during an outbreak there in 2005, 25 of those patients had symptoms that returned after at least three days without diarrhea. On average, they relapsed twice within a three-month period. The patients with recurrences included those who did and did not receive antibiotics.
For some people, recurring symptoms may be the result of an infection that never cleared, said Dr. Anurag Malani, an epidemiologist at Trinity Health Ann Arbor Hospital in Michigan. Anyone can have a lingering infection, Dr. Malani said, but in his experience, that’s been more typical in older adults or those with compromised immune systems.
People who don’t finish the full course of antibiotics also risk recurring symptoms, said Dr. Alexander Chow, the chief of infectious disease at NYC Health & Hospitals Woodhull. If people feel the drug is working after a day or two, they may decide to stop taking it — a bad idea, Dr. Chow said, since that could leave “residual parasite behind.”
For some people, a relapse in symptoms might be the result of another infection — meaning they ate contaminated food again. People don’t develop immunity to the parasite the same way they do for some viruses, said Dr. Linda Yancey, an infectious disease specialist at Memorial Hermann Health System in Houston. The parasite invades cells only in the intestinal lining, not in the tissues deeper in the body, where it would set off the production of antibodies.
“It is entirely possible to have a case of cyclospora, get over it, get better, get reinfected and get another case of cyclospora,” Dr. Yancey said, especially if you live somewhere that’s involved in the latest outbreak.
It is also possible that what seems like a return of cyclospora might instead be a lingering complication, like post-infectious irritable bowel syndrome, Dr. Chow said. In this situation, the body has cleared the parasite but residual gut damage causes further diarrhea and cramping, he said. Post-infectious I.B.S. typically resolves over time, Dr. Fisher said.
For some, the recurring symptoms have helped them realize that they need treatment to feel better.
Jill Woolley, 55, who lives in Kansas City, Mo., was initially convinced that her diarrhea, chills and body aches were from a norovirus infection. After spending two days in bed, unable to eat, she started to feel better, still nauseated but able to walk around. A week after her first symptoms, she traveled to Maine for vacation. The day she got there, her symptoms, which included debilitating stomach cramps, flared again. “I couldn’t look at a menu, couldn’t even think about food,” she said. She had read about cyclospora in the news, so she called her doctor, who recommended an antibiotic based on her symptoms.
Soon after she started the medication, she felt better. She’s been able to work out again, and to feel like her life is back to normal. “I was lucky,” she said.
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