Federal health officials say that tens of thousands of people are still infected with the E. coli strain O157:H7 each year. Ground beef has been at the center of 16 outbreaks in the past three years, and the U.S. Department of Agriculture has added more procedures and has improved training for investigators. "We are not standing still when it comes to E. coli," says Kenneth Dr. Petersen, a USDA official. Costco has opted not to rely on suppliers and instead tests for E. coli itself.

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