Matt Wise, head of the CDC’s Outbreak Response and Prevention Branch, told NBC News that he expects the number of illnesses to increase in the coming days as reports of cases come in.

“We believe this is being driven by the people who ate Quarter Pounders at McDonald’s” before it pushed them off the menu.

“For someone walking into a McDonald’s today,” he said, the risk is significantly lower.

Most of the sick people, 26, live in Colorado, including 18 in Mesa County. The older adult who died lived in Mesa County.

Six of the Colorado patients were teenagers, Dr. Rachel Herlihy, the state epidemiologist.

Nebraska has reported nine cases. Additional patients have been reported in Iowa, Kansas, Missouri, Montana, Oregon, Utah, Wisconsin and Wyoming.

The first case involving a Quarter Pounder occurred on September 27 in Colorado. State health officials alerted the CDC on Oct. 10 about an unusual increase in E. coli cases.

“We saw a larger number of cases than expected,” Herlihy said. “Some of them had eaten at McDonald’s. That’s what really drove us to put the story together.”

The CDC intensified its investigation on October 15th.

Lowell Schiller, a former Food and Drug Administration official, said it was not the first time health authorities had faced an outbreak at a major restaurant chain, citing the E. coli outbreak at Chipotle in 2015.

An open question, Schiller said, is whether the onions would be supplied exclusively to McDonald’s or go to other large restaurant chains – something he said the FDA is also likely actively considering.

Additionally, if the contamination came from a farm, which Schiller said is most likely, other products could also have been affected.

“If it went to other supermarkets or restaurants, there could be more recalls in the future,” he said.

Neither state nor federal health officials on Wednesday named a specific farm or supplier of the sliced ​​onions, although McDonald’s said it is focusing on a single supplier that supplies three distribution centers. McDonald’s declined to name the supplier.

There is no indication that diced onions on other McDonald’s menus are part of the outbreak.

The Food Safety and Inspection Service, an agency of the Department of Agriculture that oversees meat, poultry and egg products, is also working with the FDA and CDC on the investigation.

An FSIS spokesman said contamination from the quarter-pound beef patties was “very unlikely” and pointed to an FDA tracing effort that “strongly suggests” the onions.

“While the chopped onions are the likely source, FSIS continues to evaluate the safety of the ground meat used,” the spokesperson said. “A state partner collected samples of the ground beef patties for testing.”

An FDA spokesman declined to comment Wednesday, saying the outbreak was “still a rapidly evolving situation.”

The FDA said in a statement Tuesday that symptoms can include severe stomach cramps, diarrhea, fever, nausea and/or vomiting, which can occur several days after consuming contaminated food or up to nine days later. Some infections can cause severe bloody diarrhea and lead to life-threatening conditions such as hemolytic uremic syndrome, a type of kidney failure.