Eric Stelly, a resident of Greeley, Colo., is suing McDonald’s for liability, negligence, and breach of implied warranties, according to a lawsuit filed Wednesday. Stelly claims that two days after purchasing food from McDonald’s earlier this month, he experienced nausea, cramps, and bloody stools that led him to seek medical care. He tested positive for E. coli. Per the complaint, the Weld County Department of Public Health told him his food poisoning was linked to his McDonald’s meal.

“American consumers should be able to trust that the food they are served is safe from contamination,” Stelly’s attorney Ron Simon told Fortune in an emailed statement. “McDonald’s breached that trust, and will now have to earn it back.”

Stelly is one of about 50 patients sickened with E. coli linked to an outbreak in McDonald’s Quarter Pounders. One person died and ten were hospitalized from the outbreak, according to the Centers for Disease Control and Prevention. The Food and Drug Administration has launched a probe into Taylor Farms, one of McDonald’s suppliers, as the source of the outbreak, and Taylor Farms is recalling batches of yellow onions produced in a Colorado plant following the outbreak. Taco Bell, Pizza Hut, and KFC are all pulling onions from certain locations, but the company did not disclose the source of the ingredient. McDonald’s announced Friday it would stop sourcing onions from Taylor Farms’ Colorado Springs plant indefinitely.

The E. coli outbreak represents McDonald’s biggest food safety disaster in more than three decades, according to food safety lawyer Bill Marler, who plans to represent three clients in cases against McDonald’s due to the E. coli outbreak and who has previously represented Taylor Farms. Following the CDC’s announcement of the outbreak Tuesday night, McDonald’s stock plummeted as much as 9%.

Stelly’s lawsuit against McDonald’s is shocking, Marler said, because it attacks a fast-food behemoth with a generally rock-solid reputation among food safety experts. But the lawsuit more likely to make waves in the restaurant industry likely won’t come from an individual seeking personal damages—but rather from two food giants going head-to-head.

“The litigation between the victims and McDonald’s and Taylor Farms is going to pale in comparison to the litigation that is going to erupt when McDonald’s sues Taylor Farms,” Marler told Fortune. “That’s going to be a big deal.”

McDonald’s and Taylor Farms did not respond to Fortune’s request for comment.

The grand scheme of things

Taylor Farms, which employs about 20,000 people, is “a big player in the food industry,” Marler said, as it supplies pre-cut, packaged vegetables to mega-retailers like Walmart, as well as produce to chain restaurants like Chipotle. But the company has also found itself in trouble with frequent recalls and outbreaks for years, including links to some of the 600 cases of a parasitic intestinal disease cyclosporiasis confirmed in 2013, the largest outbreak of its kind since 1997.

McDonald’s next steps immediately following stymieing the E. coli outbreak will be to examine and maybe alter its supply chain, Marler said. But then—should Taylor Farms be found responsible for the outbreak—the fast-food giant will likely focus its attention on suing its onion supplier.

“It’ll be interesting to see what they do coming out of this,” Marler said. “But I gotta think they’re gonna beat the sh-t out of Taylor Farms.” 

As for a growing number of plaintiffs expected to emerge from the E. coli outbreak, only the more serious cases will likely see meaningful settlements. In instances of severe E. coli infections, such as for children hospitalized with acute kidney failure, McDonald’s could dole out $20 million to $30 to victims, Marler said. But the information known now about the outbreak suggests McDonald’s took rapid measures to reduce the spread of E. coli.

“I don’t like to discount how seriously these people are sick…but in the scheme of 30 years of experience in dealing with these cases—where sometimes the outbreaks are thousands of people—[it’s] a relatively small outbreak,” he said.