The thing to remember, vis a vis bacterial contamination, is that it's almost always the OUTSIDE of the meat that's contaminated, not the inside. With pork and chicken you're worried about an internal parasite/bacteria, which is why they're not cooked rare.
With beef, if it's grade A, it's good to go, and most everything you buy in the grocery store is fine. With tuna (which is another one people like super-rare) the story is mostly the same, though the chance of getting a parasite from fish is higher. In both cases, do a hot sear on the outside, and it'll take care of any bacteria that may be hanging around.
The reason ground beef is considered a higher threat is because the "outside" is pretty much the whole thing after the grinding process.