Beef before it's cut has the potential to have harmful microbes on it's surface, but they don't travel very fast, so they can't get deep, which is why it's safer to cook to a lesser degree than ground meat.
Ground meat on the other hand can introduce those harmful surface microbes to the rest of the meat, so burgers have to be cooked all the way through to ensure those microbes are successfully deaded.
Ground meat on the other hand can introduce those harmful surface microbes to the rest of the meat, so burgers have to be cooked all the way through to ensure those microbes are successfully deaded.