The Michigan Department of Health and Human Services has confirmed that two people died after contracting Cyclospora, a diarrhea-inducing parasite that has been spreading across the US. They are the first fatalities linked to the outbreak, which has affected at least 45 states.

The outbreak, which began in early May, has become the largest ever recorded in the country, with more than 18,000 confirmed or suspected cases, according to the latest figures from the Centers for Disease Control and Prevention (CDC). The actual number of infections is believed to be much higher because many cases remain undiagnosed. The outbreak has already far surpassed the previous record of around 4,700 cases, set in 2019.

Michigan announced the first two deaths linked to the parasite on Monday. Both victims had underlying health conditions that may have increased the severity of the illness. Cyclosporiasis, which causes severe diarrhea “with frequent and sometimes explosive bowel movements,” as well as gas, bloating, stomach cramps, nausea, and vomiting, is rarely fatal, according to the CDC.

The microscopic, spherical parasite spreads through food or water contaminated with fecal matter, particularly fruit and vegetables exposed to contaminated irrigation water. In late July, the CDC said it had identified shredded iceberg lettuce served at Taco Bell restaurants in several states as the source of the outbreak.

The agency said the supplier implicated in the outbreak, Taylor Farms de Mexico, had removed all iceberg lettuce from the US market. At the time, it was unclear whether the contaminated lettuce had been distributed to other restaurant chains or retailers.

The parasite is endemic to Mexico and other tropical and subtropical countries. There is no evidence that it can spread directly from person to person.