An E. Coli outbreak has been reported across multiple McDonald’s locations in the U.S., stemming from contaminated food or undercooked meat.

The first lawsuit was filed in Greeley, Colorado after a man began experiencing severe gastrointestinal symptoms, such as nausea, cramps and bloody stools—symptoms commonly associated with E. Coli.

The lawsuit was filed a day after the CDC issued a food safety alert warning that dozens of people fell ill after consuming the Quarter Pounder at McDonald’s.

While the exact ingredient responsible has not been confirmed, the FDA suspects the source may be the slivered onions or beef patties used in the sandwich. According to McDonald’s, these beef patties are specific to the Quarter Pounder, and slivered onions are mostly used in that sandwich alone.

In a statement with the Colorado Sun, Hope Schuler who works for The Colorado Department of Public Health and Environment shared, “We suspect that all Colorado McDonald’s locations received ingredients from the same suppliers, so we believe all McDonald’s may be affected.”

On Wednesday, McDonald’s president Joe Erlinger appeared on NBC’s Today show, reassuring the public that it’s safe to eat at McDonald’s and that the affected ingredients are likely no longer in circulation.

As of Nov. 17, McDonald’s is investing in $100 million to bring back customers after the E. Coli outbreak. The illnesses were reported between Sept. 12, and Oct. 21, in which at least 104 people got sick and 34 were hospitalized, according to federal health officials.

McDonald’s lost over $3.951 billion, a 3.94% loss for the quarter ending Sept. 30. On Wednesday, the day after the E. Coli outbreak, customer visits dropped 6.4% across the country and 24% in Colorado, where the outbreak was first announced.

McDonald’s has hope that it will win back its customers by introducing $5 meals. As of Nov. 19, McDonald’s will continue selling their Quarter Pounders in all of its restaurants.