The 49 E. coli cases have largely been concentrated in the states of Colorado and Nebraska so far, the CDC said on Tuesday.

Of the 26 people who became sick in Colorado, one older adult has died – the first, and only, death that has been linked to the outbreak.

The CDC said that an infected child is in hospital with a condition known as hemolytic uremic syndrome (HUS), which can cause kidney failure.

The first confirmed outbreak case was reported on 27 September, but McDonald’s said authorities only notified the company of their concerns late last week.

As of Wednesday, about a dozen of the people interviewed had identified eating a quarter pounder before contracting the illness.

The chain’s decision to remove the patties and onions from the menu affects stores in Colorado, Kansas, Utah, Wyoming, as well as parts of Idaho, Iowa, Missouri, Montana, Nebraska, Nevada, New Mexico, and Oklahoma.

Other hamburger items are not affected.

Burger King said on Thursday that about 5% of its restaurants receive onions from a supplier potentially linked to the outbreak. While there has been no indication of contamination or illness at those restaurants, the chain said they have disposed of the produce.