An ongoing multi-state outbreak of E. coli linked to contaminated romaine lettuce has caused horrible symptoms like bloody diarrhea, cramps, and vomiting in at least 121 people across the United States, claiming the life of one individual. Though scientists have zeroed in on Yuma, Arizona, as the source of the outbreak, they’re still not sure what actually caused the initial contamination and how it actually occurred. The dearth of information is making it hard to keep the outbreak under control.

As The New York Times reported on Monday, scientists still haven’t identified the source of the contamination itself within the Yuma growing region. The list of potential culprits is long: Romaine lettuce passes through many hands and machines on its journey from farm to grocery, and Escherichia coli, the full name of the bacteria, is a common species in the intestines of humans and animals. Most strains are benign, especially at normal levels, but the strain implicated in the current outbreak as well as in previous ones, known as E. coli O157:H7, produces the dangerous shiga toxin, which is responsible for all the horrible gastrointestinal symptoms associated with illness. Contamination with this strain is most often found in meat — the illness it causes was dubbed “hamburger disease” when the first U.S. outbreak occurred in 1982 — usually when the contents of the intestines (digested food, poop) accidentally contact the meat during butchering. But, according to the U.S. Food and Drug Administration’s Good Agricultural Practices guidelines, there are a variety of ways in which this strain can transfer from the guts of animals (or humans) to the leaves of your salad.

cows farm
E. coli O157:H7 is usually found in the intestines of animals and humans.

Chief among the FDA’s concerns is contaminated water, which can spread E. coli when it’s used to irrigate crops and apply pesticides and fertilizers and, after harvest, rinse, cool, wash, wax, and transport the produce. Then there’s fertilizer and manure, which by nature carry bacteria unless they’re properly treated with chemicals or through composting: “A particularly dangerous pathogen, Escherichia coli O157:H7, is known to originate primarily from ruminants such as cattle, sheep and deer, which shed it through their feces,” writes the FDA. And then, there’s the possibility that human workers can spread the bacteria through poor hygiene and carelessness.

The issue isn’t that officials don’t know how to prevent contamination — it’s that getting food handlers and farmers across the country to adhere to best agricultural practices has taken a long time. The government offers free voluntary audits, but it’s not clear how many farms and producers take the USDA up on this offer. In 2011, President Barack Obama signed the outbreak prevention-focused Food Safety Bill, described by CNN as the “most sweeping overhaul of America’s food safety system since 1938,” but as The New York Times explains, we have yet to see its effects because rollout and wide-scale training have been slow.

Before this year’s romaine outbreak, the biggest E. coli contamination event in the U.S. occurred in 2006, when tainted lettuce sickened 199 people in 26 states, killing three people. The source of the contamination was eventually traced back to bags of spinach from a single manufacturing facility on a particular day.

As there’s still no word on which farms or producers were affected by the contamination, the CDC insists that people avoid all romaine lettuce — that includes “whole heads and hearts of romaine, chopped romaine, baby romaine, organic romaine, and salads and salad mixes containing romaine lettuce” — from the Yuma region, as well as any lettuce without an identifiable source, until the outbreak is officially over.