A recent outbreak of E. coli tied to McDonald’s Quarter Pounder hamburgers has sickened 49 people across 10 states, resulting in one death and leaving 10 others hospitalized, according to the U.S. Centers for Disease Control and Prevention (CDC). The outbreak, which has hit hardest in Colorado and Nebraska, has also led to a severe complication in one child known as hemolytic uremic syndrome, a serious kidney condition.

The CDC issued a food safety alert on Monday, stating, "This is a fast-moving outbreak investigation." Most of the reported cases involve individuals who consumed Quarter Pounders at McDonald’s, and investigators are working swiftly to pinpoint the contaminated food ingredient. In response, McDonald’s has temporarily pulled ingredients for the Quarter Pounder from several states.

Preliminary findings from the U.S. Food and Drug Administration (FDA) suggest that slivered onions may be the source of contamination. McDonald’s has ceased using these onions and the quarter-pound beef patties in several states, including Colorado, Kansas, Utah, Wyoming, and parts of Idaho, Iowa, Missouri, Montana, Nebraska, Nevada, New Mexico, and Oklahoma.

McDonald’s issued a statement confirming that the onions came from "a single supplier that serves three distribution centers" and assured customers they are working with suppliers to restock Quarter Pounders in affected areas. In the meantime, other menu items such as cheeseburgers, Big Macs, and double cheeseburgers remain available and are not affected by the outbreak.

While the investigation continues, the FDA is working to confirm the link between the onions and the E. coli outbreak and to determine if these onions were sold or served elsewhere. The CDC has warned that the outbreak could spread to more states.

E. coli infections typically cause symptoms such as severe stomach cramps, diarrhea, fever, and vomiting within three to four days of exposure. Although most people recover within a week, severe cases can lead to kidney issues that require hospitalization. The CDC advises anyone who becomes ill after eating a Quarter Pounder at McDonald’s to seek medical attention and inform healthcare providers of their recent meal.

For now, Quarter Pounders may not be available in certain states as McDonald’s works to address the contamination issue and ensure the safety of its food supply.