This week, federal officials announced a severe E. coli outbreak linked to McDonald’s Quarter Pounder hamburgers.

The Centers for Disease Control and Prevention said on Tuesday that at least 49 people from 10 states had been sickened. Ten people have been hospitalized, including a child with hemolytic uremic syndrome, a severe complication of E. coli infection that can lead to kidney failure or even death. One person, an older adult in Colorado, has died.

Officials said that they had not yet identified the specific ingredient linked to the outbreak, but they are focused on two in particular: fresh slivered onions and fresh beef patties used for Quarter Pounder hamburgers. McDonald’s said that it had stopped using the onions and quarter pound beef patties in several states while investigators searched for the source of the bacteria.

Ground meat remains one of the most common sources of illnesses from E. coli bacteria, which are responsible for an estimated 265,000 illnesses annually. Most of these, however, are not diagnosed or tracked by the C.D.C. because people often recover on their own without visiting a doctor, said Laura Gieraltowski, the deputy branch chief of the C.D.C.’s foodborne outbreak response team.

“It’s estimated that for every one reported E. coli illness, there are likely 26 others that go unreported,” Dr. Gieraltowski said.

E. coli has also been traced to other foods this year, including organic walnuts sold in bulk bins, which sickened 13 people.