Last week, McDonald's was informed by the CDC of a possible E. coli outbreak.

• McDonald's was informed by the CDC of a possible connection between its Quarter Pounder burgers and an E. coli outbreak, according to the company.
• McDonald's removed the menu item from restaurants in the affected area prior to the CDC's advisory notice, as stated by company spokespeople.
• Currently, about 20% of McDonald's U.S. restaurants are not offering Quarter Pounder burgers.

Late last week, the Centers for Disease Control and Prevention disclosed a possible connection to an E. coli outbreak, according to company representatives.

The number of connected cases to the McDonald's Quarter Pounder burgers outbreak has increased since the initial report, though the company did not disclose the exact number of cases at that time. As of Tuesday, the CDC has attributed 49 cases and one fatality across 10 states to the outbreak.

McDonald's collaborated with the CDC, USDA, and FDA after being notified about the link. By the time the CDC issued its advisory notice, McDonald's had already decided to remove Quarter Pounder burgers from affected restaurants, as stated by spokespeople.

Currently, about 20% of McDonald's U.S. restaurants are not offering Quarter Pounder burgers.

As of Tuesday, the CDC interviewed 18 people with confirmed cases, and among them, 12 patients recalled eating a Quarter Pounder burger before becoming ill.

McDonald's is facing additional risks as it tries to win back customers who abandoned the restaurant due to years of price hikes. The outbreak only adds to these challenges, as the company hopes to entice consumers back with a $5 value meal deal.

The fast-food chain removed the Quarter Pounder from restaurants in the affected area, which includes Colorado, Kansas, Utah, Wyoming and parts of eight other states, following the CDC's advisory on the outbreak. Cesar Pina, the company's North American chief supply chain officer, made the announcement in a statement.

McDonald's burgers are cooked to an internal temperature that should kill bacteria, but the CDC is still investigating the Quarter Pounder's uncooked onions and beef patty as the potential cause of the outbreak.

McDonald's has stopped distributing onions and asked local restaurants to remove their onion supply due to contamination concerns.

The outbreak occurred between Sept. 27 and Oct. 11, during which McDonald's sold approximately one million Quarter Pounders in the affected region, according to company spokespeople.

McDonald's shares are down 5% in afternoon trading due to concerns about the impact of the outbreak on sales.