MRSA is otherwise known as methicillin resistant staphylococcus aureus. The bacteria is believed to have evolved resistance to the first few generations of antibiotics and is responsible for several difficult-to-treat infections – especially in the immunocompromised or elderly.
People can have MRSA colonization or MRSA infections. MRSA colonization is a result of having MRSA growing on your skin, nasal cavities, etc. without it causing the patient any undue stress – this is not an active infection. A MRSA infection on the other hand is characterized by fever, chills, cough, sometimes pustules and pimples. MRSA infections are life threatening and can potentially kill patients if untreated.
It appears from your question that you are colonized with MRSA and are having sores appear under your arms. To address the sores under your arms, one recommendation patients have found helpful is to avoid the use of antiperspirants and instead use only deodorants. Antiperspirants can increase the chance of sores under your arms by preventing you from sweating and clogging your pores where bacteria can grow.
MRSA rarely spreads and colonizes the blood in healthy patients – usually that occurs in patients who are immunocompromised, the very young, or the elderly. Treatment for MRSA is now usually with vancomycin in those patients who require oral or IV antibiotics.
The standard precautions to prevent spread of MRSA to others include standard hygiene techniques such as washing hands, covering your mouth when sneezing, cleaning surfaces and bathrooms and especially avoiding contact with infants, the elderly, or the immunocompromised. Precautions differ based on your colonization status so you should check with your doctor.
Signs that you have a serious infection include fever, chills, fatigue, etc. Either way, you should be evaluated by your main doctor to see how bad your infection is and treatment options based on your infection status.