Microbial contaminants can be killed by cooking, but chemical residues aren't destroyed by heat. In fact, some of these residues break down into more dangerous substances when heated, according to the FSIS National Residue Program for Cattle, a recent report by the USDA's Office of the Inspector General.
Sick dairy cows are given medications to help them recover, but if it appears an animal will die, it's often sold to a slaughterhouse as quickly as possible, in time to kill it before it dies. That way, "[the dairy farmer] can recoup some of his investment in the animal," according to the report.Meat from sick dairy cows is likeliest to end up between buns at your local fast food joint. Neurotoxins: Now you can have 'em your way.
In such cases, medications may be consumed along with the meat. Such drugs include Ivermectin (which can act as a neurotoxin in humans), Flunixin (which can damage kidneys), and penicillin (which can cause life-threatening allergic reactions in some people).