The US has had a recent surge in foodborne illnesses linked to onions. Traditionally, onions were believed to have natural antimicrobial properties and a curing process that made them resistant to contamination. Between 1998 and 2013, onions were responsible for only 161 illnesses compared to the thousands linked to leafy greens. However, in recent years, onions have been linked to four multistate outbreaks, sickening over 2,300 people, including a recent outbreak of E. coli in McDonald’s Quarter Pounders.

The origins of this contamination trend remain uncertain, though experts speculate that issues often start in the field, where factors like contaminated irrigation water or animal waste can lead to outbreaks. Processing practices, such as cutting onions into prepackaged slivers, may further spread bacteria. While cooking onions to a temperature of 165 degrees Fahrenheit eliminates pathogens, raw onions carry risks if contaminated.

Onions have historically been valued for their food safety; their papery skin  provides a natural barrier against contaminants. Additionally, curing—a drying process involving exposure to sunlight—has been shown to kill off bacteria. Despite these defenses, recent cases suggest that onions are not immune to contamination. Experts propose possible explanations, such as bacteria bypassing an onion’s protective skin through its green tops or the variance in antimicrobial strength across onion types.

However, the exact cause of the recent outbreaks remains elusive. Even with advanced technology like genome sequencing, pinpointing the source of contamination is challenging. The complexity of food safety science means that while outbreaks are relatively rare, they can have serious consequences. America’s “onion problem” reflects broader challenges in food safety, as advancements still struggle to provide complete answers and prevent foodborne illness.