If You Eat a Rare Burger
Most likely: Contamination is more likely with ground beef than it is with full cuts. However, getting sick from a burger is still fairly uncommon.
Worst case: For most healthy adults, the worst that's likely to result from eating a rare hamburger is food poisoning. At its most severe, it could land you in bed for several days with a stomach illness. The U.S. Department of Agriculture recommends that you never eat a rare burger (cooked to less than 160 degree F), since bacteria may survive in it. This is particularly important for the very young, the very old, the immunosuppressed, and pregnant women, who are all more susceptible.