The study, published in the American Journal of Public Health, found that inmates suffer from foodborne illness at a rate of 45 per 100,000 people annually, compared to only 7 per 100,000 in the general population. And 6 percent of all confirmed outbreak-related cases of foodborne illness in the United States took place in correctional institutions—significant, considering that less than 1 percent of the country’s population is incarcerated. At the same time, “desmoteric” outbreaks—the kind that occur in correctional institutions—were the country’s largest outbreaks in four of the 17 years studied. (In six other years, correctional outbreaks ranked within the top five.) Thirty-seven states reported at least one desmoteric outbreak during the same span.

What’s to blame for the dramatic rates of foodborne illness in jails and prisons? That’s harder to say. In some ways, the CDC study is highly specific about what’s making people sick: The agency determined that Clostridium perfringens and Salmonella were the most common disease-causing agents, for instance, and that tainted poultry products were the most common single culprit. But the data leave us with more questions than answers, since these raw numbers remain mostly uninterpreted. The study doesn’t cover the more systemic factors causing outbreaks in the first place.

Mariel A. Marlow, one of the study’s coauthors, was reluctant to speculate about the underlying cultural, operational, and institutional conditions leading to high rates of illness. “Oversight and regulation of correctional institutions can vary by state and institution, so just to pull out certain factors is a little difficult,” she said. The correctional system is vast and highly variable: When it comes to food, a jail in Reno may be nothing like a federal prison outside New Orleans, and a private prison in Texas may look nothing like its counterpart one county over.

But an issue this widespread still signals the existence of underlying, systemic reasons inmates are six times more likely to be sickened by their food. As it turns out, the problems that arise in correctional food service tend to have mundane roots, even if the consequences can be dramatic. Institutions struggle to enforce basic food-safety standards: Though there are reports of corruption and negligence, the primary factor appears to be that many correctional facilities aren’t equipped to execute the food-handling protocols observed in restaurants and corporate cafeterias. And when mistakes are made, there are inconsistent processes in place to ensure improvement.

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Judging from news reports, you might think the main factor causing correctional outbreaks is the poor quality of the food itself. And certainly, a slew of well-publicized lawsuits have accused correctional facilities of buying and serving dodgy ingredients. In May, for instance, a class-action suit was filed against the Oregon Department of Corrections on behalf of current and former inmates, alleging that the state-run food service is so subpar it amounts to cruel and unusual punishment. In recent years, there have been news reports of inmates served rotten chicken tacos, rancid beef, and cake that had been nibbled on by rodents. Meanwhile, earlier this year, a Michigan judge dismissed a suit brought by an inmate who said he’d been repeatedly served moldy bread and spoiled hamburger meat. (According to U.S. District Judge Gordon Quist, the complaint was without merit: In his view, the Eighth Amendment does not entitle prisoners to “tasty or aesthetically pleasing” food, only to a diet that allows them to “maintain normal health.”)