A common culprit of skin and respiratory infections, Staphylococcus aureus is highly unpredictable. Between 20 and 30 percent of people carry quiet colonies on their skin and in their nostrils, which seldom cause problems beyond the occasional rash. But in some cases these bacteria cause infections that lead to deadly complications, such as pneumonia, deep skin infections, and sepsis. Until recently, there was no way to predict which infections may take a lethal turn.