Colorado is at the center of what health officials are calling a “fast-moving” crisis deadly E. coli outbreak associated with McDonald's Quarter Pounder hamburgers.

“That means we are still trying to understand the scale and breadth 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 in the second week of October.

“The cases that we saw here in the state were really statewide. We've seen cases in the Front Range, we've seen cases in the mountains of the state and we've also seen cases in the Western.” “We've seen cases in teenagers and adults,” she said.

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

More than half of those sickened 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 most states and most menu items are not affected,” he said.

Herlihy says they are currently working with the CDC to figure out which specific ingredient is making people sick.

“This is an active part of the investigation where we are really trying to understand where the onions might be coming from. And then we continue to look at the beef and where the beef might have come from. So yeah, I'm trying to clarify, “Understand the distribution and why certain states are affected. These are all questions that we and CDC continue to ask,” she said.

Health officials say if you've recently eaten a Quarter Pounder and have any symptoms such as diarrhea, vomiting, severe dehydration and fever, you should report it to your doctor.