Why is hamburger, of all meat, particularly vulnerable to bacterial contamination such as that caused by E. coli?

When cattle are slaughtered and processed, E. coli bacteria in their intestines can get on the meat. Ground beef combines meat from many different animals, increasing the risk of contamination. E. coli bacteria on a cow's udder or on milking equipment can get into raw milk or runoff from cattle farms can contaminate fields where fresh produce is grown. Certain vegetables, such as spinach and lettuce, are particularly vulnerable to this type of contamination.