A microscopic parasite that can trigger days—or even weeks—of severe diarrhea is sickening thousands of Americans this summer, prompting health officials to investigate one of the largest outbreaks of cyclosporiasis in recent years.

The illness is caused by Cyclospora cayetanensis, a parasite that spreads when people consume food or water contaminated with microscopic parasite eggs. Unlike many stomach viruses, government officials are thinking that this year’s Cyclospora infections are most commonly linked to fresh produce, including leafy greens, herbs, berries, and other fruits and vegetables that are eaten raw.

As of mid-July, confirmed illnesses have been reported across 31 states, with Michigan experiencing the largest number of cases. Public health officials are still working to determine the exact source of the current outbreak, although investigators say lettuce and salad greens are among the foods being examined.

What Is Cyclospora?

Cyclospora is a single-celled parasite that infects the small intestine. Once swallowed, it can cause inflammation that leads to prolonged gastrointestinal illness.

Unlike norovirus or the flu, Cyclospora usually doesn’t spread directly from person to person because the parasite must mature in the environment before it becomes infectious. Instead, infections are typically traced back to contaminated food or drinking water.

Symptoms Can Take Days to Appear

One reason outbreaks are difficult to track is that symptoms often don’t begin until about one week—and sometimes up to two weeks—after exposure.

Common symptoms include:

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• Watery diarrhea
• Frequent bowel movements
• Stomach cramps
• Nausea
• Bloating
• Gas
• Loss of appetite
• Fatigue
• Weight loss
• Low-grade fever

Without treatment, symptoms can last for weeks or even more than a month, and many people experience cycles of feeling better before becoming sick again.

Who Is Most at Risk?

While anyone can become infected, some people are more likely to develop severe illness, including:

• Adults over 65
• Young children
• Pregnant women
• People with weakened immune systems
• Individuals with chronic illnesses

The greatest danger isn’t usually the parasite itself—it’s dehydration caused by persistent diarrhea.

How Is It Treated?

Doctors typically diagnose Cyclospora through specialized stool testing.

If confirmed, the infection is usually treated with the antibiotic trimethoprim-sulfamethoxazole (TMP-SMX), commonly known as Bactrim. Most healthy adults recover completely after treatment, although symptoms can linger if left untreated.

How Can You Tell If Vegetables Have Gone Bad?

One of the challenges with Cyclospora is that contaminated produce often looks completely normal. The parasite is microscopic and cannot be seen with the naked eye.

However, you should avoid eating vegetables that show signs of spoilage, including:

• Slimy or sticky surfaces
• Dark brown or black spots
• Mold growth
• Wilted or mushy leaves
• Strong sour or unpleasant odors
• Excessive moisture inside produce bags
• Discoloration that wasn’t originally present

Even if vegetables appear fresh, they can still carry harmful microorganisms, which is why proper handling is so important.

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Does Washing Vegetables Prevent Outbreaks Like This?

Yes—but only partially.

Washing fruits and vegetables under running water helps remove dirt, bacteria, pesticides, and some parasites from the surface. However, Cyclospora can cling tightly to produce, particularly leafy greens with many folds and crevices.

Experts say washing reduces risk, but cannot guarantee that every Cyclospora organism has been removed. That’s why public health officials issue recalls when contaminated products are identified.

The Right Way to Wash Fresh Produce

Food safety experts recommend these steps:

1. Wash your hands first

Scrub your hands with soap and warm water for at least 20 seconds before handling produce.

2. Rinse under cool running water

Even fruits and vegetables with peels should be washed before cutting.

3. Skip the soap

Do not wash produce with dish soap, bleach, or household cleaners. These products can leave harmful residues and are not recommended for foods.

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4. Rub while rinsing

Use your hands to gently rub firm produce such as cucumbers, apples, peppers, potatoes, and carrots while rinsing.

5. Use a produce brush when appropriate

For vegetables with firm skins, a clean produce brush can help remove dirt trapped on the surface.

6. Dry produce

Pat vegetables dry with a clean paper towel or cloth towel. Drying helps remove additional microorganisms loosened during washing.

7. Refrigerate promptly

Keep leafy greens and cut fruits refrigerated to reduce bacterial growth.

Protecting Yourself This Summer

Although health officials continue investigating the source of the outbreak, experts stress that people should not avoid fruits and vegetables altogether. Fresh produce remains an essential part of a healthy diet.

Instead, lower your risk by:

• Washing all fresh produce before eating
• Refrigerating perishables quickly
• Avoiding fruits or vegetables that appear spoiled
• Following food recall notices
• Drinking safe, clean water while traveling
• Contacting your healthcare provider if diarrhea lasts more than a few days or becomes severe

If you develop prolonged watery diarrhea, especially after eating fresh produce, don’t assume it’s a typical stomach bug. Testing and early treatment can shorten the illness and help prevent serious dehydration.