The outbreak at the center of the nation's largest known surge of the explosive diarrhea illness cyclosporiasis is over, officials say.

The Centers for Disease Control and Prevention declared the outbreak, linked directly to lettuce, as ended on Sept. 11, with a tally of 12,883 lab-confirmed cases across 21 states since May 1.

The CDC is still investigating several case clusters of the parasitic illness nationwide, but has only identified the source of one – iceberg lettuce grown by produce giant Taylor Farms in Mexico. At least five other outbreaks are still under investigation, according to the Food and Drug Administration's foodborne illness investigations page.

In total, 19,595 cases of cyclosporiasis have been reported in 49 states plus Washington, DC, as of Sept. 11, including two deaths and 1,043 hospitalizations. Thousands of additional cases are still waiting for laboratory confirmation.

The actual number of sick is likely much higher, per the CDC, as many people recover at home without getting tested. The agency's case count also has a roughly six-week lag because of the process required to confirm and officially report an infection back to the federal level. This means states can report new cases that are not reflected in the CDC count until weeks later, and new illnesses may not be tracked until over a month after someone sees a doctor.

It also means that cases will continue to be added to the CDC's count, even as the largest outbreak is considered over.

"The number of recent infections linked to the outbreak has significantly declined," the CDC's page for the lettuce outbreak said. "The contaminated lettuce linked to this outbreak is no longer available in stores or restaurants."

Cyclosporiasis, an illness caused by the parasite Cyclospora cayetanensis, results in unpleasant gastrointestinal symptoms often associated with food poisoning, such as diarrhea, nausea and vomiting. It is usually contracted through contaminated food or water, and it is not generally known to be deadly; however, it can cause more serious complications in people with compromised immune systems or other underlying health issues.

Federal authorities first began tracing case clusters in July, eventually identifying Taco Bell, which used shredded lettuce provided by Taylor Farms, as a common source among patients. Taylor Farms recalled multiple lettuce products on July 17.

A later FDA enforcement report showed that recalled lettuce was sold not just to eateries like Taco Bell, KFC and Pizza Hut, but to retailers like Walmart. But the origins of other illness clusters have yet to be identified, at least publicly. The source of a cyclosporiasis outbreak is harder to track than other foodborne illnesses because of a long incubation period, difficulty obtaining samples and the inability to use the same genetic tracking tools available for pathogens like E. coli and listeria. Because it is usually not fatal and not as common in the U.S., it is also less researched than other pathogens, something experts told USA TODAY they hope to see change.