Why is rare hamburger a food poisoning risk but not rare steak?

Rare hamburger is more of a risk then steak for one basic reason: the pathogens become internalized inside the ground up beef.

When an animal is slaughtered for food, the contents of its digestive tract may contaminate the meat. If this happens then bacteria like E. coli that may be in the digestive tract are deposited on the external portions of the meat. While the meat processing industry does much to prevent this, it still does occur. Therefore several other procedures are in place such as thorough washing and sanitizing of the meat before it is further processed.

While safeguards are in place, either through cross-contamination in the food processing plant, or by survival of E coli despite best efforts, there may remain traces of E coli on the meat. When the meat is cut into steaks or other whole cuts, the E coli survive on the surface of the steak, but are easily killed when the surface of the meat reaches at least 155 degrees. Frying, grilling or roasting steaks will easily kill E coli, as it is exposed on the surface. But, if the meat that had E coli was ground into hamburger we have a different problem. In this case the E coli become imbedded in the interior of the ground meet where they are protected from the external heat source such as a grill, a flame or an oven. For heat to penetrate into the middle of a hamburger, proper cooking must place, so that the meat is not overcooked on the outside, but the center gets to 155 degrees. This is the problem, in order to ensure bacteria are killed in hamburger; one must use a proper thermometer and test the most internal part of the hamburger, passing the thermometer in through the side. The temperature must read 155 degrees for 15 seconds before we are sure E coli is killed. One only needs to make sure that the surfaces of steaks are seared. Hamburger poses some difficult problems because of the internalization of E coli and the fact that most people do not use a thermometer when cooking hamburgers.