Well first of all everyone has staph living on their skin. The vast majority of staph strains are not harmful... however some are. Of these strains there are also some which are antibiotic resistant and can only be treated with a few very powerful antibiotics. A lot of people that get these antibiotic resistant strains such as MRSA get it while in the hospital. The reason being that MRSA lives in hospitals and patients usually have a compromised immune system. You can see why this could be problematic for infection control.
Here's a link to a pretty good site that has some useful information about antibiotic resistant infections: http://www.* removed
Hope that helps