The Centers for Disease Control and Prevention said Wednesday that the number of infected people from the E.coli outbreak linked to McDonald’s Quarter Pounder hamburgers has increased to 90 from 75.

The outbreak was first reported on Oct. 22 and on Wednesday, CDC said 27 people have been hospitalized due to the outbreak, which has already killed one person.

Two of them developed hemolytic uremic syndrome (HUS), a serious condition that can cause kidney failure, as of Oct. 24.

The E. coli O157:H7 strain that led to the McDonald’s outbreak is said to cause “very serious disease,” especially for the elderly, children and people who are immunocompromised.

The FDA noted that symptoms begin anywhere from a few days after consuming contaminated food or up to nine days later.

The symptoms include severe stomach cramps, diarrhea, fever, nausea, and/or vomiting.

As of Wednesday, the outbreak has affected Colorado, Kansas, Utah, Wyoming, Iowa, Missouri, Montana, Nebraska, New Mexico, Oregon, Wisconsin, Washington and Michigan. Colorado has reported the highest infections with 29 people who have fallen sick.

Initial findings by McDonald’s and the FDA showed the outbreak was likely a result of the slivered onions used in the Quarter Pounder and sourced by a single supplier that serves three distribution centers.

The company along with the Colorado Department of Agriculture also ruled out the possibility of beef patties being a source of the outbreak, leaving the onions as the most likely cause for the outbreak.

McDonald’s said that it would resume selling Quarter Pounders burgers this week after it temporarily took the item off the menu in a fifth of its 14,000 US restaurants that were impacted.

On Tuesday, company executives brushed off any potential sales hit from the outbreak with CEO Chris Kempczinski apologizing to customers and adding that he was “confident in the safety of eating at McDonald’s.”

The health regulator and the company confirmed that Taylor Farms was the supplier for the affected locations.

Taylor Farms has recalled several batches of yellow onions produced in a Colorado facility, according to a recall memo on Wednesday by US Foods, one of the largest US suppliers of food service operations.