http://www.nytimes.com/2009/10/04/health/04meat.html?_r=2&pagewanted=1&hp

Highlights: meat from different parts of cows and even from different slaughterhouses. These cuts of meat are particularly vulnerable to E. coli contamination

Those low-grade ingredients are cut from areas of the cow that are more likely to have had contact with feces, which carries E. coli

then cleaned with ammonia

Yay McDonalds