An outbreak of cyclosporiasis, a gastrointestinal illness caused by the parasite Cyclospora, continues to unfold across multiple U.S. states.

The illness is challenging for health officials to track for several reasons, including a lag between exposure to the parasite and symptom onset, as well as a lack of testing for the illness, which can lead to underreported case counts.

Thus far, more than 1,600 Cyclospora cases have been linked to shredded iceberg lettuce served at Taco Bell in five states, prompting Taylor Farms to recall all iceberg lettuce sourced from central Mexico that was distributed to restaurants and retailers. Health officials say they are also investigating other outbreaks in other states that are unrelated to the Taylor Farms cases.