Avoid the most, and worst, contamination by buying grass-fed, organic beef. The best method for doing this is to buy directly from a local rancher or through a CSA. Search on http://www.localharvest.org/ to find one near you.
Second best: ask the butcher to grind up a roast for you. That way you know your hamburger came from just one animal.
There Is Poop in Basically All Hamburger Meat | Mother Jones.