One person is dead and 48 others across 10 states have been sickened in an E. coli O157:H7 outbreak that appears to be linked to McDonald's Quarter Pounders and the slivered onions used on the burgers, according to the Centers for Disease Control and Prevention.

McDonald's has paused distribution of the slivered onions and removed Quarter Pounders from the menus of restaurants in areas known to be affected. As of now, those areas include Colorado, Kansas, Utah, and Wyoming, as well as portions of Idaho, Iowa, Missouri, Montana, Nebraska, Nevada, New Mexico, and Oklahoma.

However, the CDC was quick to note that the size and span of the outbreak are likely larger than is currently known. "This outbreak may not be limited to the states with known illnesses, and the true number of sick people is likely much higher than the number reported," the agency said in its outbreak notice posted Tuesday afternoon.

The agency notes that many people sickened by E. coli-tainted food will recover without medical care and will not be tested for the bacterial pathogen, meaning those illnesses will never be linked to the outbreak. Additionally, for cases that do require medical care, there are delays in linking illnesses to outbreaks. The CDC notes that it usually takes 3 or 4 weeks to connect an illness to an outbreak, making it likely that recent illnesses may not be counted yet.

Still, the CDC also notes that this appears to be a "fast-moving outbreak." Of the 49 cases in 10 states linked so far, three were identified as recently as October 11. Overall, the illness onsets span from September 27 to October 11.

The swift identification is likely aided by the fact that sickened people have unusually clear memories of what they ate. "Everyone interviewed has reported eating at McDonald’s before their illness started, and most specifically mentioned eating a Quarter Pounder hamburger," the CDC reported. Food recall surveys done amid outbreaks don't always reveal such a clear picture.

The specific source of the E. coli outbreak strain—an E. coli O157:H7, a Shiga toxin-producing E. coli (STEC)—has not been determined, the CDC said. But investigators have narrowed their focus to the fresh quarter-pound beef patties and slivered onions used on the popular Quarter Pounder. The CDC notes that the quarter-pound beef patties are only used on Quarter Pounders, and the slivered onions are "primarily" used on Quarter Pounders.