Cyclospora cases have been reported in at least 47 states this year, and investigators have continued to probe how many could be linked to a common contaminated source.
The Centers for Disease Control and Prevention said the number of states tied to the deadly outbreak had grown from nine to 15.
Arkansas, Iowa, Missouri, Nebraska, New Hampshire and North Carolina were added to a list that previously included Illinois, Indiana, Kansas, Kentucky, Michigan, Ohio, Oklahoma, Pennsylvania and West Virginia.
Investigators initially narrowed in on lettuce served at Taco Bell. Federal officials subsequently said they are focused on Taylor Farms as the source of the lettuce in the outbreak, and the company has recalled iceberg lettuce grown in central Mexico.
There are several reasons cyclospora outbreaks can be hard to investigate and contain, said Dr Juan Luis Marquez, medical director of the Washtenaw County and Livingston County health departments in Michigan.
It can take weeks between when someone is exposed to the parasite and when they come down with symptoms; it's harder to analyse the parasite in laboratories; and it's a lot of work to interview everyone who is sick and clearly identify common exposures, Dr Marquez said.
Also, "there have been staffing cuts at all levels of public health - federal, local, state - so there are less resources and capacity generally available," he said in a statement.
Cyclospora is a microscopic, spherical parasite that commonly causes watery diarrhoea "with frequent and sometimes explosive bowel movements," according to the CDC.
Outbreaks tend to occur most often in the late spring and summer.
The heat-loving parasite infects the bowels and spreads through faeces. The illness, called cyclosporiasis, is less common than foodborne illnesses caused by other germs, including salmonella and E. coli. Many cyclospora cases are never linked to a specific food or other source.
The worst year in the US for infections was 2019, when about 4700 illnesses were reported nationally. This year has far surpassed that.
Michigan alone has reported more than 12,000 - including two deaths. Adding in the other states that have reported cases, the national total stands at over 25,000.
The FDA says it is tracking five separate ongoing cyclospora outbreaks.
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