Cows that are fed grass (their natural diet) have intact intestinal flora which enhances their immunity. And their meat has been shown to be not only nutritionally superior, but clean & uncontaminated as well. Conversely, the corn fed cows tend to be sicker, have more inflammation and their meat is high in Omega 6 fatty acids which cause inflammation in humans who consume their meat.
Contamination is usually introduced during slaughter and packaging due to substandard sanitation, and is not inherent in the neat itself.