Staphylococcus bacteria commonly reside on human skin and mucus membranes; they even live in soil. They're usually harmless. However, hospital patients, especially those with weakened immune systems, can be susceptible to skin infections from Staphylococcus. These staph infections can cause rashes, boils and other skin problems, and the infection can spread to the bloodstream [source: Mayo Clinic].
Staph infections are a serious concern, but the real problem comes from bacterial strains that are resistant to many antibiotics and thus extremely difficult to treat. Methicillin-resistant Staphylococcus aureus (MRSA) is the most infamous. It causes skin infections, but it also shows up under other categories on this list, like respiratory infections and surgical site infections. With few treatment options, MRSA can be life-threatening to someone with a compromised immune system.
Years of overuse and improper use of antibiotics are the major reasons these resistant strains have become so prevalent. That's why one of the CDC's big initiatives is something the agency calls "antimicrobial stewardship," an effort to ensure antibiotics are used properly and in a way that won't lead to more resistant strains of antibiotics [source: CDC].