An E. coli outbreak that has affected 75 people is linked to McDonald's Quarter Pounders, according to the CDC.

• Since the agency announced the outbreak on Tuesday, there have been 49 cases and one death across 10 states.
• Officials are closely scrutinizing the shredded onions in Quarter Pounders as a possible contaminant.

The Centers for Disease Control and Prevention announced on Friday that a deadly E. coli outbreak linked to McDonald's Quarter Pounders has resulted in 75 cases across 13 states, as they continue to investigate the source of the spread.

One older adult has died and 22 people have been hospitalized due to the outbreak in Colorado.

Of the 61 patients with available information, 22 have been hospitalized and two have developed a serious condition that can lead to kidney failure, known as hemolytic uremic syndrome. Among the 42 people interviewed by the CDC, all reported eating at McDonald's, and 39 of them said they ate a beef hamburger, the agency stated.

According to the CDC, the age range of those infected with the disease ranged from 13 to 88. The agency emphasized that the number of cases in the outbreak is likely much higher than what has been reported so far. Additionally, the CDC stated that the outbreak may not be limited to the states with related cases. Many patients don't test for E. coli and recover from an infection without receiving medical care, the CDC said. It also usually takes three to four weeks to determine if a sick person is part of an outbreak.

Since Tuesday, the CDC has reported 49 cases and one death related to the outbreak across 10 states. Meanwhile, the restaurant chain's shares have dropped by 6%.

McDonald's did not immediately respond to a request for comment from CNBC.

Billions of dollars are generated annually by McDonald's through its core menu item, Quarter Pounder hamburgers.

McDonald's has halted the distribution of slivered onions in the affected region after health officials suspect them as a possible contaminant in Quarter Pounder burgers. Restaurants in the area have been instructed to remove the slivered onions from their supply.

The CDC reports that McDonald's stores in several states, including Colorado, Kansas, Utah, Wyoming, Idaho, Iowa, Missouri, Montana, Nebraska, Nevada, New Mexico, and Oklahoma, have temporarily stopped using Quarter Pounder slivered onions and beef patties.

McDonald's has identified Taylor Farms as the supplier of the sliced onions it removed from its supply chain. Taylor Farms has issued a recall on four raw onion products due to potential E. coli contamination. McDonald's has pulled onions from select restaurants in response to the outbreak.

Federal agencies are also probing the Quarter Pounder's beef patty as a possible suspect.

McDonald's has removed Quarter Pounders from restaurants in the affected areas as the CDC and other federal agencies work to control the outbreak. Approximately one-fifth of McDonald's U.S. restaurants are not selling Quarter Pounder burgers.

McDonald's representatives stated on Wednesday that it is currently uncertain if the outbreak is affecting the number of customers visiting its restaurants.

The company will release its third-quarter earnings on Tuesday and may provide additional information to investors during the conference call.

McDonald's is experiencing an outbreak after several quarters of sluggish U.S. sales. Due to price-sensitive consumers, there has been a decline in restaurant visits, prompting McDonald's and other fast-food chains to introduce value meals to increase sales. Wall Street analysts predict that the company will report U.S. same-store sales growth of 0.5% for the third quarter, according to StreetAccount estimates.

McDonald's is striving to alleviate customer concerns by affirming the safety of its menu offerings for consumption.