More people have been confirmed as infected with E. coli in an outbreak traced to McDonald’s Quarter Pounder hamburgers.

There are now 75 patients, up from 49 earlier this week. One of the patients has died, according to the Centers for Disease Control and Prevention. Thirteen states now have patients, up from 10 previously reported. Of 61 patients with information available, 22 have been hospitalized. Two patients have developed hemolytic uremic syndrome, a serious condition that can cause kidney failure.

Of the 42 patients interviewed, all reported becoming sick after eating at McDonald’s. The most recent person to become ill had illness onset on Oct. 10.

The Food and Drug Administration is investigating the outbreak and has determined that slivered onions are the most likely source of the E. coli O157:H7 that is making people sick.

Taylor Farms, the supplier of fresh onions to McDonald’s, has initiated a recall of its yellow onions. The onions were distributed by US Foods, the country’s largest supplier of food and supplies to food service operators and were also sent to other food service customers. 

McDonald’s has stopped serving Quarter Pounder sandwiches in the outbreak states. It has also stopped using slivered onions, according to company officials.

The number of sick people is likely to increase because for every one patient identified in an E. coli outbreak there are 26 who go undiagnosed. This is because some people do not seek medical attention and others are not specifically tested for E. coli.

Patients in this outbreak live in Colorado, Kansas, Utah, Wyoming, Idaho, Iowa, Missouri, Montana, Nebraska, Nevada, New Mexico, Oklahoma, Michigan, Wisconsin, Washington and Oregon, according to a map from the CDC.