The skin offers the most contact area for an infection, though intact skin is impermeable to many would-be invaders. Wounds present an infection opportunity.
Many pathogenic bacteria cannot survive long on healthy skin because of direct inhibitory effects of lactic acid and fatty acids in sweat, as well as sebaceous secretions that generate an acidic environment.
The grape-shaped Staphylococcus aureus, the most common cause of staph infections, is an exception to being averse to an acidic environment. About 20% of the human population carry Staph aureus for much of their lives. Staph aureus can be commensal, but can also cause a range of illnesses, from minor skin infections to life-threatening maladies such as pneumonia.
Pathogenic Staph infections have been a perennial problem in hospitals since the beginning of the antibiotic era. With much exposure and experience, most strains of hospital Staph aureus have acquired antibiotic resistance. A few strains can now resist all clinically useful antibiotics.