Staphylococcus aureus is another common type of bacteria whose harmfulness depends on the strain. Some kinds of S. aurea bacteria can cause serious skin infections, commonly referred to as staph infections. Methicillin-resistant Staphylococcus aureus (MRSA), for example, is a rather nasty version of the pathogen that causes painful skin boils and is transferred by skin contact. It's typically spreads in hospitals and other health care settings, a process that can be quickened via dirty cell phones [source: Mayo Clinic].
In a 2009 study, Turkish microbiologists found that more than half (52 percent) of the 200 health care workers' mobile phones that they tested were contaminated by Staphylococcus aureus, and that 38 percent of those phones had been exposed to the methicillin-resistant strands of the bacteria [sources: Ulger et al., Mayo Clinic].