Michigan is the hot spot for an outbreak of cyclosporiasis, an infection caused by a parasite that can cause weeks of watery diarrhea.

The infection has been reported in 45 states since May 1, according to state health department data.

According to Georgetown’s Health Security Operations Center, which tracks data at both the state and national levels, more than 8,260 cases of cyclosporiasis have been reported in the U.S., with the bulk of cases spread across the Midwest and Mid-Atlantic states.

The U.S. Food and Drug Administration on Thursday identified shredded iceberg lettuce from a supplier in Mexico that was sent to Taco Bell locations in five states as a source of the outbreak, but other food sources are likely driving the spread. A source close to the federal investigation confirmed to Business Insider that the supplier is Taylor Farms, one of the largest salad companies in North America.

Business Insider created a map showing the number of cases in each state. It is based primarily on data reported by state health departments, either from their official channels or in statements to local media.

Such data was not immediately available in Georgia, Rhode Island or Utah, so the range of cases provided by the Centers for Disease Control and Prevention was used for them.

States with the best-known cases of cyclosporiasis include Michigan and Ohio.

The high number of confirmed cases in Michigan (more than 4,300) does not necessarily mean the outbreak started there.

“It more likely means that Michigan has better resources for testing, reporting and epidemiological work, so more cases are detected and counted,” public health expert Katelyn Jetelina wrote in her newsletter, Your Local Epidemiologist, on July 11.

As of Thursday, Ohio (1,409), New York (510), Indiana (327) and North Carolina (307) were reporting more than 300 confirmed cases.

How to avoid Cyclospora, which has been linked to chopped lettuce

The Michigan health department said Monday that ongoing investigations suggest “lettuce or salad greens” are a possible source of contamination, but other foods cannot be ruled out.

Cyclospora is spread through contaminated food and water, and symptoms, such as watery diarrhea, nausea, and stomach cramps, can take up to two weeks to appear, making it difficult to trace a definitive source.

The parasite is resistant to disinfectants and washing food does not guarantee its elimination, since it adheres especially well to agricultural products.

Health authorities say to permanently kill the parasite, cook food to at least 158 ​​degrees Fahrenheit. Buying whole heads of lettuce and peeling the outer layer of leaves instead of choosing pre-washed, bagged, or pre-mixed salads is also a good option, if you’re more risk-tolerant.

According to the CDC, the typical cyclospora season lasts until the end of August and cases could continue to increase until then.