Staph infections, caused by the bacterium staphylococcus aureus, are increasing in frequency and are more difficult to treat with antibiotics. many staph infections are now caused by mrsa, and methicillin-resistant strain of s. aureus. what explains the more widespread phenomenon of mrsa prevalence in staph infections? a) vaccines for mrsa have become not effective because of changes to human immune systems. b) the presence of antibiotics induced the bacterial genomes to mutate and develop resistance to the drug. c) decreased hospital sanitation practices to save money have resulted in increased presence of resistant staph infections. d) selective pressures by excessive antibiotic use have increased the frequency of resistant strains of s. aureus over the last few decades.