The invention provides a method for preventing or treating a disease caused by an extracellular microorganism, said method comprising systemically administering to a subject in need thereof a prophylactically or therapeutically effective amount of a salicylic acid (SAL) or a SAL analog. The extracellular microorganism can be of the bacterial genus Staphylococcus, for example, Staphylococcus aureus. The extracellular microorganism can be a strain that is resistant to at least one antibiotic. The strain can be selected from the group consisting of methycillin-resistant Staphylococcus aureus (MRSA), vancomycin-intermediate Staphylococcus aureus (VISA) and vancomycin-resistant Staphylococcus aureus (VRSA). The invention also provides a method for preventing or treating an infectious disease caused by of methycillin-resistant Staphylococcus aureus (MRSA), vancomycin-intermediate Staphylococcus aureus (VISA) or vancomycin-resistant Staphylococcus aureus (VRSA), comprising systemically co-administering in a synergistic combination to a subject in need thereof prophylactically or therapeutically effective amounts, individually or collectively, of a salicylic acid (SAL) or a SAL analog and at least one additional antimicrobial agent, for example, vancomycin and/or linezolid.