As you may have seen in news headlines or social media memes, there’s explosive diarrhea going around. Understanding how the immune system responds to the parasite that causes the unfortunate disease called cyclosporiasis can help explain why the infection is mild in some people but can cause severe or long-lasting illness in others.

The 2026 outbreak of cyclosporiasis in the U.S. has sickened over 4,100 people as of July. Public health officials have traced a major cluster of illness to contaminated produce imported from Mexico.

As immunologists, we see cyclosporiasis as an interesting example of how the immune system influences disease. Unlike many bacteria that release toxins that damage body tissues, the parasite that causes cyclosporiasis infects the cells lining the small intestine and lives inside them. Symptoms of the infection, including explosive diarrhea, are caused mainly by the body’s immune response to the parasite.

As a result, the severity and duration of infection depend largely on how well the immune system controls it.

How your immune system fights cyclosporiasis

Cyclosporiasis is caused by a microscopic parasite named Cyclospora cayetanensis. When a person ingests Cyclospora eggs by consuming contaminated food or water, the tough protective wall of the parasite breaks down in........

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