Colorado is at the center of what health officials say is a "fast moving" and deadly E. coli outbreak linked to McDonald's Quarter Pounder hamburgers.
"It means that we're still trying to understand the scope and breadth of this particular outbreak," said Rachel Herlihy, State Epidemiologist at the Colorado Department of Public Health and Environment.
Herlihy says the first case in Colorado was reported during the second week of October.
"The cases that we've seen here in the state have really been across the state. We've seen cases on the Front Range, we've seen cases in the mountains in the state and we've also seen cases on the Western Slope. We've seen cases in teens to adults," she said.
According to the U.S. Centers for Disease Control and Prevention, the outbreak now stretches across 10 states with a total of 49 cases and one death.
More than half of those sick are from Colorado, and Herlihy says the death occurred on the Western Slope.
McDonald's believes the source of the contamination could be slivered onions or Quarter Pounder beef patties -- both are specific to that one menu item at the restaurant.
Joe Erlinger, president of McDonald's USA, shared a video message on Tuesday saying food safety is a priority.
"It is important to note that the majority of states and majority of menu items are not affected," he said.
Herlihy says they are currently working with the CDC to determine which specific ingredient is making people sick.
"That's an active part of the investigation is really trying to understand where the onions may have come from. And then also continuing to look at the beef, and where the beef may have come from as well. So yeah, trying to clarify that, understand distribution, why certain states are affected. All of those are questions that we and CDC are continuing to ask," she said.
Health officials say if you ate a quarter pounder recently and have any symptoms including diarrhea, vomiting, severe dehydration and fever to report to your doctor.