Colorado is at the center of what health officials say is ‘fast-moving’ deadly E. coli outbreak paired with McDonald’s Quarter Pounder burgers.

“It means we’re still trying to understand the breadth and scope of this particular outbreak,” said Rachel Herlihy, state epidemiologist with the Colorado Department of Public Health and Environment.

Herlihy says the first case in Colorado was reported the second week of October.

“The cases we have seen here in the state have occurred across the state. We’ve seen cases in the Front Range, we’ve seen cases in the mountains across the state and we’ve seen cases out west as well. Slope. We have seen cases in teenagers and adults,” she said.

The outbreak now extends across 10 states with a total of 49 cases and one death, according to the U.S. Centers for Disease Control and Prevention.

More than half of the sick are from Colorado, and Herlihy says the deaths occurred on the Western Slope.

McDonald’s believes the source of the contamination could be chopped onions or Quarter Pounder beef patties – both of which are specific to that one menu item at the restaurant.

Joe Erlinger, president of McDonald’s USA, shared a video message Tuesday saying food safety is a priority.

“It is important to note that the majority of states and most 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: trying to understand where the onions might be coming from. And then also continuing to look at the beef, and where the beef might be coming from as well. So yeah, I’m trying to clarify that, understand the distribution , why certain states are affected. These are all questions that we and the CDC continue to ask,” she said.

Health officials say if you have recently eaten a quarter pounder and have symptoms including diarrhea, vomiting, severe dehydration and fever, you should report it to your doctor.