Because it is ground, hamburger meat has a lot of surface area for bacteria to cling to. Plus when it's processed, meat from many different animals blends together, increasing the changes of the beef containing contagions.
One way to avoid ground beef that could contain a variety of bacteria is to buy irradiated meat. Keep your homemade patties thin, so they cook all the way through. The internal temperature of the meat should be 160 degrees. When you are out, order your burger well done.