Infectious Diseases
Cyclosporiasis outbreak: What to know
Maya Brownstein
July 17, 2026
Cyclosporiasis, a parasitic gastrointestinal illness, is spreading across the U.S., sickening thousands and hospitalizing hundreds. Below, Bill Hanage, professor of epidemiology at Harvard T.H. Chan School of Public Health, shares what’s important to know about the disease and this atypically large outbreak
Update: On July 16, the U.S. Centers for Disease Control and Prevention posted a notice warning against consuming shredded lettuce served at Taco Bell in five states. While news reports indicate that the agency has linked cases to a California-based supplier, this has not been officially confirmed.
Q: What is cyclosporiasis?
A: Cyclosporiasis is an illness characterized by watery diarrhea, caused by the protozoon parasite Cyclospora cayetanensis. It typically spreads through contaminated water or food, especially leafy greens like lettuce or cilantro. Importantly, the Cyclospora parasites shed during infection are not immediately infectious, so direct person-to-person spread is not driving this outbreak, in contrast with something like norovirus
Cyclosporiasis usually resolves on its own but can be prolonged, lasting weeks in some cases. And it can follow a relapsing course, meaning people feel better for a while before getting sick again. Cyclosporiasis can become dangerous when someone infected becomes severely dehydrated from diarrhea. A very small proportion of cases require hospitalization. For treatment, the antibiotic trimethoprim-sulfamethoxazole (brand name Bactrim or Septra) is effective, alongside supportive care to prevent dehydration.
Q: Is this illness always lurking? What’s behind the current outbreak?
A: There have actually been many cyclosporiasis outbreaks, they’ve just not been at this scale. They tend to occur during spring and summer, as warm conditions are required for Cyclospora parasites to mature to the point of becoming infectious
This year, cyclosporiasis cases began to tick up in late spring, notably in the Northeast. We’re now seeing extremely rapid growth in the Midwest, especially Michigan, producing an outbreak of record size. In fact, the outbreak is certainly even larger than headline figures suggest, because most cases of diarrhea don’t get tested. It is also a nationwide problem, and the apparent epicenter in Michigan likely reflects more testing and attention in that state as opposed to more cases
We don’t yet know the specific source—or sources—of this current outbreak. Outbreak investigators interview people who have fallen ill, trying to figure out what they might have eaten in common that could have led to them becoming infected. But cyclosporiasis has an incubation time of about a week to ten days. Can you remember what you ate a week ago? The most commonly contaminated items are also frequently found in lots of different meals—lettuce is in burgers, burritos, side salads, etc.—and have long, complicated supply chains, making it harder to track them.
Q: What’s challenging about this outbreak, and what’s needed to get it under control?
A: To get it under control we need to identify the source, and that is hard. It is also true that public health in the U.S. is chronically overburdened. There are too few outbreak investigators, and those we have are under-resourced and overworked. Once a potential source has been identified, it must be promptly communicated. Michigan officials have recently pointed to lettuce, which would fit with previous outbreaks. Given that this is a national problem it would benefit from coordinated communication through the Centers for Disease Control and Prevention, which has so far been lacking.
Q: How might the outbreak continue to unfold, and what should people keep in mind for their own health and safety?
A: Even if the outbreak’sd will continue to fall ill for a week or so, reflecting cyclosporiasis’ incubation period. And given the scale of the outbreak in numbers and geographic spread, there may well be multipleuntil August. As the weather gets cooler in September, that will start to prevent the parasite from maturing and will turn the odds in our favor
I’d encourage people to remember that, despite all the news you’re likely seeing, cyclosporiasis is still uncommon. You are far more likely to get norovirus. That said, if you’ve been having watery diarrhea on and off for weeks, it might be cyclosporiasis and you should give your doctor a call
About The Author
Maya Brownstein
Media Relations Manager in the Office of Communications
Related Topics
Infectious Diseases
Last Updated
Featured in this article
Bill Hanage
Professor of Epidemiology
Get the latest public health news
Stay connected with Harvard Chan School
Subscribe to our newsletters
